A)

OK, that means that I have to give the users some time before archiving new messages.

What are useful times: 30 days?

I than have to archive first all messages older than 30 days.

All other archiving runs should look for messages 30-31 days old, correct?
Or are there other possibilities to limit the scan range?

There is a catch to a long delay, because you generally don't want email to be deleted before archiving, and that is readily enforceable. This results in growing trash bins that get users' attention. (I've done email work for the day and empty the trash bin, but it came back)

There really should be a way for Retain to update where a message is finally filed,  and I last had that discussion pre Ideas area that it was being considered. I'm going to have to dig a bit more to see where we've ended up and if we need an Idea on this (or if someone gets to it before I do, I have other things ahead of it in my queue)