I haven't thought about it much yet. I guess one solution that isn't very good but would work is to just make it possible to manually choose the encoding when entering a password. This would also help for files where the encoding is guessed wrong.
But that requires updates to the UI resources and translations, so I'll wait until I have some other changes to apply too, it's a bit of a bother to get everyone coordinated to do an UI update.
Good to hear!
Can you check if you can make a file with 3.0.1 that will not open in The Unarchiver? If you can, that would be very useful. Try different compression methods and see if one of them doesn't work.