I don’t have the instrument at hand, but:
1. To the best of my recollection, the moisture is expelled from the melodion when you push the button and blow. I wouldn’t worry about condensation damaging the instrument. If you’re concerned about this possibility, simply store it on end with the mouthpiece hole down.
2. Disassemble on a towel so as to catch any small nuts that slip out of their slots when you remove the bots/screws. You’ll be able to see where they were seated, where they’ll remain unless they fall out when you turn it over.
3. If it is airtight, I would not try to do anything to the hairline crack. Better to just leave it as is. With anything that might affect airtightness, I’d just leave it alone if there’s not a problem.
Good luck with this; it’s a nice instrument.