I think you're making it a bit too complex, but that may be because of my limited view on your actual situation.
The date of the last transaction has a relation to the reload time of the qvd. Newer transactions aren't in there because the QVD was reloaded before the transactions were created. If you just want to know how to incrementzally update the data in a QVD or a related table, you can use the QVD reload timestamp. There is a script function for that to get the timestamp from the QVD header (no loading the entire qvd): QvdCreateTime(filename).