1) I don't know what you mean by "properly". But the answer is probably yes. You can still fill in the backup to reserve slot too. And don't choose Let coach decide and such things. If you asking whether these three players get simillar minutes it is hard to tell, it would differ a bit game by game, influenced by stamina, fouls, possible injuries and difference between quality of a starter and his backup.
2)Yes, exactly. EVen in the same game - you can check these minutes in >players>Weekly stats
3)It is some team from your conference. It is important for fan surveys and for attendance. It influences these things more than just regular games. It is (probably) randomly drawned. Notice that each team with a human manager has a rival but its rival can have different team as a rival (or no rival at all if it is a bot). Even more teams* can have the same rival (and the rival can have completely different rival other than them).
*I bet no more than two.