I can't tell specifically what you need, but I suggest using the "standard" approach to adjust a datetime to a given boundary.  For example, the code below totals by MINUTE,...

As to the query plans for these:

SELECT A.FirstName, A.LastName, B.Location
FROM TableA A
LEFT OUTER JOIN TableB B
ON A.PersonId = B.PersonId AND B.Location = 'London'

Yes, the E:\ paths will be the same, if...

You should also look at tablediff.exe to do that. 

Yeah, being a command-line utility it's a little quirky to get used to, but I've found it works well...

Use:
-t ","

instead of:
-t "|"

I'm guessing the error was about conversion to int, based on your comments, but it would be nice to be sure.
If so, run a pre-audit on the source data...


SELECT
  @StartDt = DATEADD(HOUR, 6, DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()) - 1, 0)),
  @EndDt = DATEADD(HOUR, 1, DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0))

MERGE is notorious for some performance issues. You might want to try "UPSERT" -- an equivalent UPDATE followed by an INSERT.

For overall performance,, make sure you have...

100% not using a JOIN clause screams "noob".

But I see many more issues.

A serious one is not including a schema name before the table names. ...