When you load the data, it ends up in RAM whether you directly connect to the database or use a QVD. QVDs can be useful as they can be used in multiple documents and be reloaded multiple times without going back to the database.
You can remove all the data from the file and therefore from RAM by choosing: File>Reduce Date>Remove All Values. This will decrease the file size, but in order for the document to be of any use, the user would have to do a Reload to get the data back.
You can also use DROP TABLE TableName; in the script to remove a table from memory.
To answer your question in the last paragraph, yes. Reduce Data to Remove All Values and Save. Then when the document is open, create a macro to reload the document (ActiveDocument.Reload).
The QVW stores your layout objects and data. As you add more layout objects (charts, list boxes, etc) the file should increase slightly. Data will probably represent the majority of your QVW file size.
I'm not sure I understand the last question. When you open a QVW that contains data, I believe the data is immediately placed into RAM. If you have Reduced the data or if you didn't save the app with data in it, then no data will be loaded into RAM until you reload.