They're not stupid questions, but unfortunately I only know as much as players have been told by devs/staff. Yes, Rotom's different forms will be attainable. I have absolutely no idea where the inter-region boat will be, I can only speculate like Stickbow did in his post. And LOL time window. Uh, Soon™ is all you're going to get but if Kyu's statement about Sinnoh taking less time than Unova did ends up being true we can expect it within a couple months. c: Or given how things go here, like another 6 months. Who knows? Do we? Do the devs? Hmmmmm
It's been confirmed that Nasty Plot will be breedable onto Misdreavus upon Sinnoh's release and extrapolating from that as well as the fact that Spiritomb isn't a legendary, I'd say that the answer is yes with 99.3% confidence.
From everything I've seen so far, I don't think any catchable Pokemon will have hidden abilities when Sinnoh comes out. Those are being saved for a future update that will introduce co-op dungeons and legendary Pokemon as well as Hidden Abilities.
No one knows, could be with Sinnoh, could be later. Could be released with Sinnoh, but bugged, so removed, fixed and then re-implemented later.
You have to realize that coding is an insanely messy job and it's really hard to make sure everything's working right and identify why the things that don't work aren't working. So if you're not patient and understanding, you'll just end up being frustrated. And once Sinnoh is entirely implemented, a lot of features will still probably be bugged, broken or missing. On top of that, the small dev team probably have irl things to do (like give their waifu pillows some attention), and work on this game on their free time, so things might take a while.
@Kyu did promise a Q&A on forums though, so if you see this tag, pls tell us if that'll be a thing soon(™).
Disclaimer though: I'm not a staff member and nothing I say is an official answer, it's just stuff I know from multiple conversations staff and players have had on forums in the past.