You are expected to do them in order and this is the tradeoff for not having to slog through one four hour dungeon each week.

You also didn't actually "earn the ability to do P11S" really; each week you are required to reclear 9 and 10 to unlock 11 again. You can bypass this requirement by joining a party where the leader has cleared 9 and 10, and they can pull you into 11 with them. Since you didn't actually have the right to enter 11 and are entering on someone else's 9 and 10 clears, you forfeit your right to enter 9 and 10 for rewards that week.