1. I always wash my sets, works fine (unless they have stickers). However, after that is done, dry them with a towel, don't simply leave them to dry themselves, as that way they can get water stainds on the plastic.2. Have you tried simply washing the paint off or scrubbing it until it comes off? If that doesn't help, maybe use turpentine, but as I'm not sure about the safety of that, make sure to try it on some scrap plastic to make sure it won't damage it.3. BrickLink would be a good place to buy missing parts.4. Sorry to say it, but I'm pretty sure there's no way to return colour to those.