UPDATE:
As a last ditch effort, I was at a local Yamaha shop for something else and got talking with their lead tech about my lower unit. I explained everything I did but he said he felt confident that he could get it apart. I figured, what the heck - give it a shot! A week later he called me and said "OK, I see what you were talking about, now!" He said he never came across one this hard. But we agreed to go ahead and keep going - he called a day later and said he had it about 1" apart - which is more than I ever got.
He said the upper splines of the driveshaft/crank seemed solid - which, in my mind, made continuing with this effort worthwhile. With that in mind, I said go ahead and keep going. I also dropped off my spare lower unit and our thought was that if it didn't all of sudden start to come off, then go ahead and destroy the existing lower and put a slide hammer on the remaining driveshaft stub. That's what he did. The spare lower is rebuilt and on the engine and he said the test ride went well... other than dropping RPM at high RPM's... but I already knew that
Anyways, just wanted to follow up that it's off. Although considering most people don't have a spare lower unit laying around, it's probably not a technique that most would use to get it off!