I agree with your comments, except the ones about The 501st. They used to accept custom mandos until 2005.
I forget the reason why, but I do know why, and I can't explain the reasoning without taking a while to do so.
But if The 501st did accept custom mandalorians, as said earlier, then A bunch of low quality costumes would want to join, and no doubt some would get accepted into the Club, it is bound to happen. They don't want that. Sith Lords go through Spanos, BH's go through...somewhere, and things of that sort. It is hard to find a low quality SL in The Legion, because Spanos is in charge of that, and he does a graet job of it.