You are right, that last part isn't a glitch, Betsy the dragon has a treasure bag (I'm playing post moonlord, expert).
I can definitely see what you're saying. I'm using one of the new accessories for the extra sentry and wearing Stardust helm and leggins with the Valhalla breastplate, just switched to that from full Stardust. If the armor wasn't so much higher in defense I'd stay full stardust, I love my minions, but with the other classes getting sentry bonuses and added melee/magic/range bonuses with the other sets, a bonus minion (not sentry) or two wouldn't be too OP, IMHO. Though on the flip side, the armor does give a bigger damage bonus to the minions. I'm sure the devs have weighed it all, they're really good at keeping things in balance, but still