But: "I have tried the trick where you open the shutter to cock the lens through the back, but it's a no go."
The trick with the other screw is explained here (the "hard jamming" bit).
This works by moving the front key assembly out of the way of the lens, so it can be removed from the camera.
The front key assembly of course needs to be put back in its proper place. The page suggests counting turns. May work, but as you can imaging, is not very accurate. So try at your own peril.
Also, getting the lens off is one thing. Correcting a possible fault that caused the 'jam' is another thing. Something that this obviously does not do.
I've gotten some stubborn jams when the lens-to-body mating is a bit loose. A CLA was required for both to reliably kill the problem.
Basically, it was just loose enough to create the half-cocked condition when the male flange didn't properly engage the female slot on the lens. A little bit of play between the two can cause havoc because it will engage then disengage and then engage again, etc., repeat.
...I managed to get the lens off by really working the inside screw and having another person hit the release and twist the lens at the same time. So, it's off -that's the good news. The bad news is my camera is messed so I'm sending it to Vancam this week for a CLA and repairs if needed 'cause now the camera won't wind and the shutter curtains are sluggish.