I must first compliment you on your courage to do something like this with an Access DB, which from my experience is very difficult to do anything SQL-like. Anyways, on to the review.

First join

Your IIF field selections might benefit from using a Switch statement instead. It seems to be sometimes the case, especially with things SQL, that a SWITCH (more commonly known as CASE in typical SQL) is quite fast when just making simple comparisons in the body of a SELECT. The syntax in your case would be almost identical, although a switch can be expanded to cover a large chunk of comparisons in one field. Something to consider.

  SWITCH (
    expr1, val1,
    expr2, val2,
    val3        -- default value or "else"
  )

A switch can also help readability, in larger statements. In context:

  MAX(SWITCH(B.XTStamp <= A.RecTStamp,B.XTStamp,Null)) as BeforeXTStamp,
  --alternatively MAX(-(B.XTStamp<=A.RecTStamp)*B.XTStamp) as BeforeXTStamp,
  MIN(SWITCH(B.XTStamp>A.RecTStamp,B.XTStamp,Null)) as AfterXTStamp

As for the join itself, I think (A.RecTStamp<>B.XTStamp OR A.RecTStamp=B.XTStamp) is about as good as you're going to get, given what you are trying to do. It's not that fast, but I wouldn't expect it to be either.

Second join

You said this is slower. It's also less readable from a code standpoint. Given equally satisfactory result sets between 1 and 2, I'd say go for 1. At least it's obvious what you are trying to do that way. Subqueries are often not very fast (though often unavoidable) especially in this case you are throwing in an extra join in each, which must certainly complicate the execution plan.

One remark, I saw that you used old ANSI-89 join syntax. It's best to avoid that, the performance will be same or better with the more modern join syntax, and they are less ambiguous or easier to read, harder to make mistakes.

FROM (FirstTable AS A INNER JOIN 
  (select top 1 B1.XTStamp, A1.RecTStamp 
   from SecondTable as B1
   inner join FirstTable as A1
     on B1.XTStamp <= A1.RecTStamp
   order by B1.XTStamp DESC) AS AbyB1 --MAX (time points before)

Naming things

I think the way your things are named is unhelpful at best, and cryptic at worst. A, B, A1, B1 etc. as table aliases I think could be better. Also, I think the field names are not very good, but I realize you may not have control over this. I will just quickly quote The Codeless Code on the topic of naming things, and leave it at that...

“Invective!” answered the priestess. “Verb your expletive nouns!”

"Next steps" query

I couldn't make much sense of it how it was written, I had to take it to a text editor and do some style changes to make it more readable. I know Access' SQL editor is beyond clunky, so I usually write my queries in a good editor like Notepad++ or Sublime Text. Some of the stylistic changes I applied to make it more readable:

• 4 spaces indent instead of 2 spaces
• Spaces around mathematical and comparison operators
• More natural placing of braces and indentation (I went with Java-style braces, but could also be C-style, at your preference)

So as it turns out, this is a very complicated query indeed. To make sense of it, I have to start from the innermost query, your ID data set, which I understand is the same as your First Join. It returns the IDs and timestamps of the devices where the before/after timestamps are the closest, within the subset of devices you are interested in. So instead of ID why not call it ClosestTimestampID.

Your Det join is used only once:

The rest of the time, it only joins the values you already have from ClosestTimestampID. So instead we should be able to just do this:

    ) AS ClosestTimestampID
    INNER JOIN SecondTable AS TL1 
        ON ClosestTimestampID.BeforeXTStamp = TL1.XTStamp) 
    INNER JOIN SecondTable AS TL2 
        ON ClosestTimestampID.AfterXTStamp = TL2.XTStamp
    WHERE ClosestTimestampID.XmitID IN (<limited subset S>)

Maybe not be a huge performance gain, but anything we can do to help the poor Jet DB optimizer will help!

I can't shake the feeling that the calculations/algorithm for BeforeWeight and AfterWeight which you use to interpolate could be done better, but unfortunately I'm not very good with those.

One suggestion to avoid crashing (although it's not ideal depending on your application) would be to break out your nested subqueries into tables of their own and update those when needed. I'm not sure how often you need your source data to be refreshed, but if it is not too often you might think of writing some VBA code to schedule an update of the tables and derived tables, and just leave your outermost query to pull from those tables instead of the original source. Just a thought, like I said not ideal but given the tool you may not have a choice.

Everything together:

SELECT
    InGPS.XmitID,
    StrDateIso8601Msec(InGPS.RecTStamp) AS RecTStamp_ms,
       -- StrDateIso8601MSec is a VBA function returning a TEXT string in yyyy-mm-dd hh:nn:ss.lll format
    InGPS.ReceivID,
    RD.Receiver_Location_Description,
    RD.Lat AS Receiver_Lat,
    RD.Lon AS Receiver_Lon,
    InGPS.Before_Lat * InGPS.BeforeWeight + InGPS.After_Lat * InGPS.AfterWeight AS Xmit_Lat,
    InGPS.Before_Lon * InGPS.BeforeWeight + InGPS.After_Lon * InGPS.AfterWeight AS Xmit_Lon,
    InGPS.RecTStamp AS RecTStamp_basic
FROM (
    SELECT 
        ClosestTimestampID.RecTStamp,
        ClosestTimestampID.XmitID,
        ClosestTimestampID.ReceivID,
        ClosestTimestampID.BeforeXTStamp, 
        TL1.Latitude AS Before_Lat, 
        TL1.Longitude AS Before_Lon,
        (1 - ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) 
            / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp))) AS BeforeWeight,
        ClosestTimestampID.AfterXTStamp, 
        TL2.Latitude AS After_Lat, 
        TL2.Longitude AS After_Lon,
        (     (ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) 
            / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp)) AS AfterWeight
        FROM (((
            SELECT 
                A.RecTStamp, 
                A.ReceivID, 
                A.XmitID,
                MAX(SWITCH(B.XTStamp <= A.RecTStamp, B.XTStamp, Null)) AS BeforeXTStamp,
                MIN(SWITCH(B.XTStamp > A.RecTStamp, B.XTStamp, Null)) AS AfterXTStamp
            FROM FirstTable AS A
            INNER JOIN SecondTable AS B 
                ON (A.RecTStamp <> B.XTStamp OR A.RecTStamp = B.XTStamp)
            WHERE A.XmitID IN (<limited subset S>)
            GROUP BY A.RecTStamp, ReceivID, XmitID
        ) AS ClosestTimestampID
        INNER JOIN FirstTable AS Det 
            ON (Det.XmitID = ClosestTimestampID.XmitID) 
            AND (Det.ReceivID = ClosestTimestampID.ReceivID) 
            AND (Det.RecTStamp = ClosestTimestampID.RecTStamp)) 
        INNER JOIN SecondTable AS TL1 
            ON ClosestTimestampID.BeforeXTStamp = TL1.XTStamp) 
        INNER JOIN SecondTable AS TL2 
            ON ClosestTimestampID.AfterXTStamp = TL2.XTStamp
        WHERE Det.XmitID IN (<limited subset S>)
    ) AS InGPS
INNER JOIN ReceiverDetails AS RD 
    ON (InGPS.ReceivID = RD.ReceivID) 
    AND (InGPS.RecTStamp BETWEEN <valid parameters from another table>)
ORDER BY StrDateIso8601Msec(InGPS.RecTStamp), InGPS.ReceivID;

• Adicionado atributos adicionais e condições de filtro.
• Qualquer forma de junção cruzada é eliminada usando consultas aninhadas mínimas e máximas. Este é o maior ganho de desempenho.
• Os valores de flanco mínimo e máximo retornados pela consulta mais aninhada interna são valores de chave primária (varreduras) que são usados ​​para recuperar atributos de flanco adicionais (lat e lon) usando uma busca por cálculos finais (o acesso tem um equivalente de aplicação).
• Os atributos primários das tabelas são recuperados e filtrados na consulta mais interna e devem ajudar no desempenho.
• Não há necessidade de formatar (StrDateIso8601Msec) o valor de hora para classificação. Usar o valor de data e hora da tabela é equivalente.

Planos de execução do SQL Server (porque o Access não pode mostrar isso)
Sem a ordem final porque é caro:
Clustered Index Scan [ReceiverDetails].[PK_ReceiverDetails] Custo 16%
Clustered Index Seek [FirstTable].[PK_FirstTable] Custo 19%
Clustered Index Seek [SecondTable].[PK_SecondTable] Custo 16%
Clustered Index Seek [SecondTable].[PK_SecondTable] Custo 16%
Clustered Index Seek [SecondTable].[PK_SecondTable] [TL2] Custo 16%
Clustered Index Seek [SecondTable].[PK_SecondTable] [TL1] Custo 16%

Com a ordem final por:
Ordenar Custo 36%
Varredura de Índice Clusterizado [ReceiverDetails].[PK_ReceiverDetails] Custo 10% Busca de
Índice Clusterizado [FirstTable].[PK_FirstTable] Custo 12%
Busca de índice clusterizado [SecondTable].[PK_SecondTable] Custo 10% Busca de
índice clusterizado [SecondTable].[PK_SecondTable] Custo Busca de índice clusterizado de 10% [SecondTable].[PK_SecondTable] [TL2] Custo Busca de índice agrupado
de 10%
[SecondTable].[ PK_SecondTable] [TL1] Custo 10%

Código:

select
     ClosestTimestampID.XmitID
    --,StrDateIso8601Msec(InGPS.RecTStamp) AS RecTStamp_ms
    ,ClosestTimestampID.ReceivID
    ,ClosestTimestampID.Receiver_Location_Description
    ,ClosestTimestampID.Lat
    ,ClosestTimestampID.Lon
,[TL1].[Latitude] * (1 - ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp))) + [TL2].[Latitude] * ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp)) AS Xmit_Lat
,[TL1].[Longitude] * (1 - ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp))) + [TL2].[Longitude] * ((ClosestTimestampID.RecTStamp - ClosestTimestampID.BeforeXTStamp) / (ClosestTimestampID.AfterXTStamp - ClosestTimestampID.BeforeXTStamp)) AS Xmit_Lon
    ,ClosestTimestampID.RecTStamp as RecTStamp_basic
from (
        (
            (
                select
                     FirstTable.RecTStamp
                    ,FirstTable.ReceivID
                    ,FirstTable.XmitID
                    ,ReceiverDetails.Receiver_Location_Description
                    ,ReceiverDetails.Lat
                    ,ReceiverDetails.Lon
                    ,(
                        select max(XTStamp) as val
                        from SecondTable
                        where XTStamp <= FirstTable.RecTStamp
                     ) as BeforeXTStamp
                    ,(
                        select min(XTStamp) as val
                        from SecondTable
                        where XTStamp > FirstTable.RecTStamp
                     ) as AfterXTStamp
                from FirstTable
                inner join ReceiverDetails
                on ReceiverDetails.ReceivID = FirstTable.ReceivID
                where FirstTable.RecTStamp between #1/1/1990# and #1/1/2020#
                and FirstTable.XmitID in (100,110)
            ) as ClosestTimestampID
            inner join SecondTable as TL1
            on ClosestTimestampID.BeforeXTStamp = TL1.XTStamp
        )
        inner join SecondTable as TL2
        on ClosestTimestampID.AfterXTStamp = TL2.XTStamp
    )
order by ClosestTimestampID.RecTStamp, ClosestTimestampID.ReceivID;

Teste de desempenho da minha consulta em relação à consulta que contém a junção cruzada.

FirstTable was loaded with 13 records and SecondTable with 1,000,000.
The execution plans for my query didn't change much from what has been posted.
Execution plans for the cross join:
Nested Loops Cost 81% using INNER JOIN SecondTable AS B ON (A.RecTStamp <> B.XTStamp OR A.RecTStamp = B.XTStamp
Nested Loops drops to 75% if using CROSS JOIN SecondTable AS B' or ',SecondTable AS B
Stream Aggregate 8%
Index Scan [SecondTable][UK_ID][B] 6%
Table Spool 5%
Several other Clustered Index Seek and Index Seeks (similar to my query as posted) with Cost of 0%.

Execution time is .007 and 8-9 seconds for my query and the CROSS JOIN.
Cost comparison 0% and 100%.

I loaded FirstTable with 50,000 records and a single record to ReceiverDetails for a join condition and ran my query.
50,013 returned between 0.9 and 1.0 second.

I ran second query with the cross join and allowed it to run for about 20 minutes before I killed it.
If the cross join query is filtered to return only the original 13, execution time is again, 8-9 seconds.
Placement of the filter condition was at inner most select, outer most select and both. No difference.

There is a difference between these two join conditions in favor of the CROSS JOIN, the first uses a predicate, the CROSS JOIN does not:
INNER JOIN SecondTable AS B ON (A.RecTStamp <> B.XTStamp OR A.RecTStamp = B.XTStamp) CROSS JOIN SecondTable AS B

Adding a second answer, not better than the first but without changing any of the requirements presented, there are a few of ways to beat Access into submission and appear snappy. 'Materialize' the complications a bit at a time effectivity using 'triggers'. Access tables do not have triggers so intercept and inject the crud processes.

--*** Create a table for flank values.
    create table Flank (
         RecTStamp      datetime not null
        ,BeforeXTStamp  datetime null
        ,AfterXTStamp   datetime null
        ,constraint PK_Flank primary key clustered ( RecTStamp asc )
        )

--*** Create a FlankUpdateLoop sub. (create what is missing)
    -- loop until rowcount < 5000 or rowcount = 0
    -- a 5K limit appears to be manageable for Access, especially for the initial population.
    insert into Flank (
         RecTStamp
        ,BeforeXTStamp
        ,AfterXTStamp
        )
    select top 5000 FirstTable.RecTStamp
        ,(
            select max(XTStamp) as val
            from SecondTable
            where XTStamp <= FirstTable.RecTStamp
            ) as BeforeXTStamp
        ,(
            select min(XTStamp) as val
            from SecondTable
            where XTStamp > FirstTable.RecTStamp
            ) as AfterXTStamp
    from FirstTable
    left join Flank
        on FirstTable.RecTStamp = Flank.RecTStamp
    where Flank.RecTStamp is null;

--*** For FirstTable Adds, Changes or Deletes:
    delete from Flank where Flank.RecTStamp = CRUD_RecTStamp
    execute FlankUpdateLoop --See above. This will handle Adds, Changes or Deletes.

--*** For SecondTable Adds, Changes or Deletes:
    --delete from Flank where the old value is immediately before and after the new flank value.
    --They may or may not get be assigned a new value. Let FlankUpdate figure it out.

    --execute deletes for both beforextstamp and afterxtstamp
    --then update flank

    delete *
    from flank
    where beforextstamp between (
                    select min(beforextstamp)
                    from flank
                    where beforextstamp >= '3/16/2009 10:00:46 AM'
                    ) and (
                    select max(beforextstamp)
                    from flank
                    where beforextstamp <= '3/16/2009 10:00:46 AM'
                    );

    delete *
    from flank
    where afterxtstamp between (
                    select min(afterxtstamp)
                    from flank
                    where afterxtstamp >= '3/16/2009 10:00:46 AM'
                    ) and (
                    select max(afterxtstamp)
                    from flank
                    where afterxtstamp <= '3/16/2009 10:00:46 AM'
                    );

    execute FlankUpdateLoop

--*** Final Report Query***--
    --Should execute without issues including 'deferred execution' problem.
    --Add filters as needed.
    select FirstTable.XmitID
        ,FirstTable.ReceivID
        ,ReceiverDetails.Lat
        ,ReceiverDetails.Lon
        ,BeforeTable.Latitude * (1 - ((FirstTable.RecTStamp - BeforeXTStamp) / (AfterXTStamp - BeforeXTStamp))) + AfterTable.Latitude * ((FirstTable.RecTStamp - BeforeXTStamp) / (AfterXTStamp - BeforeXTStamp)) as Xmit_Lat
        ,BeforeTable.Longitude * (1 - ((FirstTable.RecTStamp - BeforeXTStamp) / (AfterXTStamp - BeforeXTStamp))) + AfterTable.Longitude * ((FirstTable.RecTStamp - BeforeXTStamp) / (AfterXTStamp - BeforeXTStamp)) as Xmit_Lon
        ,FirstTable.RecTStamp as RecTStamp_basic
    from (((
        FirstTable
    inner join Flank on FirstTable.RecTStamp = Flank.RecTStamp)
    inner join SecondTable as BeforeTable on Flank.BeforeXTStamp = BeforeTable.XTStamp)
    inner join SecondTable as AfterTable on Flank.AfterXTStamp = AfterTable.XTStamp)
    inner join ReceiverDetails on FirstTable.ReceivID = ReceiverDetails.ReceivID
    order by FirstTable.RecTStamp;