1. yes, you get the flags back. 100% positive on that. (happened to me)
2. definitely contact the gms first. if they approve it, just be aware you can't bid on each others' players & things like that.
3. should be fairly easy to get in a private league
4. most PLs i've been in only had 8 teams & 1 or at most 2 rounds of playoffs. so anything like that will end before your supporter runs out. anyway, not even sure if you can join a PL that ends after your supporter runs out? i'd think if that ever happened the games would have to be played anyway, just for the sake of the rest of the league...
5. yes, 100% positive. (same as #1) basically, all the supporter feature are just hidden when it runs out. & come back when you buy it.