Ok firstly, Congrats on Supporter. Ive had it for over a year and love it! lots of cool features to play with =^^=
A. it means that the league will be longer than 3 months (probably 16 teams in it) and that your Supporter will expire before the end of the league. You can join/create a shorter PL that will fit into your 3 month trial, or alternatively purchase a longer supporter. This is done so that teams don't "exit" the PL midseason when their supporter runs out.
B. Not sure what you mean here by being the season being over a month long. Each season is 14 weeks long. Supporter runs independently of that. If you buy supporter for 3 months, it will expire exactly 90 days after you submit payment, regardless of the time of season in the game. Is that kind of what you are asking?
Hopefully this helped, best of luck this season!
-NS