"Jason Bourne" wrote
The bottom half came off uneventfully.
The top half is another story.
I believe the drive shaft splines are corroded to the drive plate.

More likely it's stuck in the gimbal bearing.

I know the potential for damage with the "hammer
mechanics" approach and am looking for a finesse
method, possibly from someone more experienced.

This may not sound very finesse-ish, but I got it from a merc mechanic
friend. Lift the upper unit up on its pivots and let it drop down to the
stop. Repeat as necessary. If gravity alone doesn't do the trick, give it a
little push, kind of slamming it down and being very careful to apply just
enough force to not break something. (Nobody knows exactly how much force
that is.) If you get any movement, drive a couple of wedges in between the
drive and bell housing to keep the pressure on as you continue. Failing
that, proceed as you suggested by cutting off the bellows. If the penetrant
doesn't work, you could try applying heat, but that's definitely dangerous.
I'm told this is how the professionals do it, that there's no magic
procedure or tool. If you're reasonably competent mechanically and you can't
get it off without breaking something, a real mechanic probably couldn't
either.