It should work just fine to load from different Access databases or excel or any other source, but first, you need to make sure you are using the correct CONNECT string.
Use the Connect button in the Script Editor window to help you with the string. In any QVW file, you can use as many CONNECT strings as you need, be it one or one hundred.
Hope that helps.
the problem with hanging reload, I had with SQL( firebird odbc connector) too. There were 2 reasons, slow connection to the server it seemed just like hanging but could be executed with a lot of patience. On the other side there were some of my sql statements, which created no errors but didn't seem to work with odbc/firebird db. I don't know if something of this helps for ms access. Its just my experience with connection to databases in QV.
For your last Question this link might be helpful?!
But in general I would extract/store the data in several qvd files first(with some (pre)cleanup if necessary) and then load/consolidate them in one app. I'm not that advanced in qv yet, but this helped me a lot to avoid errors(and help debugging), avoiding datamess and reduce complexity.