If you run out of clan/course league matches to simulate, you will simulate other random matches, so they do end up getting skipped for a while then. We use a strong preference for simulating your own leagues because most people simulating aren’t in any leagues, so this balances it out a bit, plus you can more directly cause games you’re waiting for to show up in your rankings (because you are part of your league).
If you quit your clan, it should remove those leagues from your multiplayer sessions, but it doesn’t yet until you resubmit the sessions after leaving the clan.
I think you will just fight yourself once for every time you submit due to that particular clan that just has you now.
Old sessions are from players that don’t care as much about the rankings (or they would be resubmitting to increase their score). Players who have just submitted are waiting for rankings, so we simulate more games for them so that things update faster.
It turns out that it is slow to pick things randomly based on complicated queries like “score is near mine”, plus you may not play players too far below or above you, and it will be hard for your score to change. And on levels with huge numbers of players, score within +/- 200 places may be very, very similar. We do grant more score based on the difficulty of the player you beat relative to your own strength, so playing players similar to you doesn’t change your rank as much a beating someone far above you, and that makes it take a long time for good strategies to converge.