I got the door out. For those interested in how it happened, read on:
Recap: Driver door stuck in open position, would not slide along lower track runner. As such, I was not able to remove the door since the door must be able to travel fully rear-ward (99.9% closed) so it can be swung outward and clear the forward fender.
As suggested here by Terry & Joe, I removed fasteners for the upper track runner which completely freed the door from the body. However I was still stuck since the lower track runner simply not clear the rear door jam. I could not see how to get the whole assy out without radically bending the track runner and I did not want to do that.
So taking Joes next suggestion, I took a long, strong piece of thin wood and tried to release any mechanical linkage that might be holding the door in the open position. With the door moving freely left / right & up / down (as limited as it was) within the fender, I could get a good view deep in the pocket, toward the front. Frankly I saw nothing mechanical that appeared to be holding this door open.
I examined the passenger door which was laying on my workbench and I saw nothing that would explain why the door was stuck open. Back at the car, I saw, deep in the pocket, a mouse nest. It was right on top of the lower track runner /axle assy. More and more I thought "This can't have anything to do with any sort of mechanical linkage. It has to be an organic third-party influence."
I took a chance and decided to use brute force. I placed a thin piece of wood to protect the aft jam fiberglass and, grabbing the door firmly, sharply smacked the lower track runner against piece of wood protecting the aft jam.
Sure enough, after three sharp raps, the door broke free from the lower track runner and now slid freely. Now I could bring the door full aft and remove the whole assy.
The jam was purely organic in origin. After years of mouse urine adulterating the parts, the lower runner was fused to the lower track. And as you can see from these pictures, the track is terribly corroded. I'm not sure I can salvage the lower track runner, but I'll see how it looks after cleaning it up. May need to get one fabricated.