First of all, We want to tell you that we have resolved the APK download issues bug.
As for your questions:
If I understood the APK behavior right: I can have geometry in ‘neutral zone’ outside levels (that will be downloaded at the application start) and geometry inside levels (that will be downloaded when each level is initiated). If this is right then it’s great. I can have universal parts of levels outside levels and non-universal parts in levels.
Yes, you understood right
Do I have to destroy geometry through function before the next level loads or will the previous-level geometry be automatically cleared from the cache when next level starts?
The objects aren't destroyed when switching levels, they just appear or disappear. The cache is not beeing cleaned either.
Does geometry outside levels stay throughout the entire game-session?
Yes