re: Scalloped crater rims?
Robert, I'm not a geologist and it'd be really cool if this crater turned out to be volcanic but I have to tell you my first impression and all subsequent ones have convinced me it's an impact crater. The scallops are unusual I'm certain but this crater is obviously very old and weathered and this crust we've found all over the area certainly could've eroded unevenly as we're seeing.
I think your suggestions certainly fall under quality brainstorming though and no possibilities should ever be thrown out.
Art