I finally got the trim off the tub valve and discovered the problem. This is the flange that sits on top of the tub surface. The valve goes through this and has a 'lip' that sits on top of this. A nut is run up from underneath to secure the valve to the tub's surface. The trim/handle assembly has a recess this piece fits in to when the trim is snugged down. If you look at the tome of this piece, you can see a small deformation. I think this deformation got hung up in the recess on the trim and wouldn't allow the trim to spin free when I went to unscrew it.