Hmmm.....did this problem ever get solved, and if so, how?
(The solution might be useful to someone else, someday.)
The problem was solved, but not by me. I was totally unable to do it myself. Luckily for me a retired master machinist volunteered his services. He not only took apart the seized components, but adjusted and made everything right—some things he made more than right, by improving on the original design. I'm incredibly lucky and grateful.
By the time I sent it to him the part was moving slightly. It turned out that in addition to being seized, it was galled from a set-screw, which prevented me from pulling it apart. I'm not sure what he used (I'm sure no sodium hydroxide!), but he put it in a machine and was able to easily separate the stuck parts. He then machined everything and repaired a bit of damage. It now works great.
I'm glad to hear that the problem has been solved. I was just wondering if you had ever managed to get the piece apart.
If you aren't part of the solution, you must be part of the precipitate.