SUCCESS, ladies and gents!
Long story short, I think hammering on the m/c is what did it...
The extended version, I used a conical grinding bit on the rotary tool, to dig enough of a groove to get some blows in with the chisel. (Never got close enough to the threads to make a "seam" there). Was getting frustrated at it still not coming loose, turned my attention back to hammering on the m/c from the inside, peeked my head back around and saw that there was more clearance now at the top stud! Then gave it a yank, and voila!
Thanks again for all your help and suggested approaches!