If you switch to using Go BACK UP instead of Go UP then when you enter a directory and then exit it by parenting out again, Opus will leave the directory selected and will also remember the calculated folder sizes (since both things are remembered by the lister's history/cache).
This FAQ explains how to switch over to Go BACK UP in all the different places (toolbar, hotkey, file display border):
[HOW TO: Remember scroll positions and file selections)
That should solve questions 1 and 2.
For question 3 I think you'll need to send GPSoft a feature request as, at the moment, folder sizes don't get updated automatically after changes to the folders.