All enemies are allowed on that wave.
Fester is not part of the randomizer – it only appears manually. Are you talking about adding it to the randomizer’s “saucer” tier, so it has a chance of appearing on any wave? That’s a big change. Poll it.
Because that’s the Meteor Storm boss, which appears from 0%
There’s no safe zone, that’s the whole point
Yes, intended. In Comet Chases the screen never stops.
When you say “not in steam version”, do you mean that you haven’t tried it in Steam version, or you have tried it but the music works correctly there?
Fixed in v.84 
Bug . This particular bug dates back to CI3 (c)2006.
Banned.
Acknowledged, will fix.