Success!
PBB freed up the wedge to move down. The aluminum stem was still fused to the steel steerer. Soaking with PBB, alternating heat and cold, and soaking in ammonia did not help. I used a hacksaw blade to saw inside the stem so it was partially split with 4 separate cuts. I then compressed it with pliers, and reapplied the PBB. I was then able to twist it out.

Thanks to all for your help!