It worked! Took about 30 minutes to cut all the metal out around the O-ring working with hacksaw and vise grips. Used a second pair to grip the outer tailpiece and the inner came out with it. Extra lucky!
Wire brushed it out and I could see the retainer wafer (edges circled). At this point it looked crudded up, but I put the key in to see if it would spin and it did!
I left the key in at the neutral position and tried to figure out how to depress the retainer wafer. Tried my pocket knife, which worked. It stuck in, likely due to the crud, and I was able to push from the back and it popped out. Tapped it on the table a few times and the retaining wafer started working again. Just gonna clean it up and try popping it in the new cap.
Thanks for the help everyone!
