In fact the thing about mindflayer souls seems like a really philosophically interesting, thematically rich topic the likes of which all the best CRPGs grapple with, and I'm particularly sad that it's glossed over.
I believe it goes against the lore. While BG3 is my first game from the BG series, I am pretty sure it was clearly stated that you must have a soul to cast magic. And there ARE wizards among mindflayers.
Crown
We don't get to decide what to do with the crown? It just falls in the river and that's it, aside from a few lines for Gale? It seemed like it was set up to be a much bigger deal than that. I was sure we'd get to choose to give it to Mystra or Raphael or destroy it, or claim it for ourselves, maybe in some kind of elaborate secret ending where we ascend to godhood like in a certain other CRPG.
You have to play as Gale as your Main Character to get proper ending on this.