You are as frustrated as I was when I finally found the valve stuck in the valve body by the piece of chrome plating that flaked off(see my previous post). Did you carefully check over the valve body and the tiny valves to make sure that debris from the failed piston did not hang one of them up? The B2 is downstream from the filter so any debris generated in that chamber can go to the valve body where operating clearances are very tight.. If the B2 chamber is not getting any pressurized fluid when in D, the piston will not move.

I admit this is a long shot, but you seem to have covered the more likely scenarios, just as I had.

Peter