Welcome back! ^_^
1 yes. The core book, is the BRB for want of a better term. The other rules are for other game types. There are also some suppliment books that have more rules for factions, but that doesn't affect us too much.
2 yes, but the Repressor is now a "Legends" unit, i.e. can't be used in tournaments. Our Codex came at the literal end of 8th ed.
3 Not sure, I still haven't got the 9th ed rules and organised myself yet.
4 Old models and their base sizes are completely valid. GW has said many times that a GW model can still be used, with its original size base.
5 The regulars are still about, just not the focus any more.
Just so you know, Kill Team has been updated and Sisters have rules for it, albeit not that great. I've forgotten which book they're in though.
Good to see you here again! ^_^