There’s no solution for changing the rests on a condensed staff that I can think of, no. Perhaps somebody cleverer than me has an idea. But I can’t think of anything better than saying that you might need to fix this by editing a resulting PDF in a graphics editor at the end of the process, I’m afraid.
For the time signatures, unfortunately what’s needed is a feature that doesn’t yet exist in the software, which is a means of defining two global time signatures and assigning instruments to either of them, so that in fact you don’t need to use any local time signatures at all. This is something that we plan so that it’s easier to handle polymetric and indeed polytempo music, but it’s not something that is possible today, so you’re quite right that using global time signatures isn’t possible, since those global time signatures will end up “interrupting” the open metered music in the other instruments.
You can certainly create text that looks like time signatures if that is helpful – see e.g. this recent thread.