I decided to proceed using some brute force. After removing the 5 nuts, I punched the protruding bolts with a rubber hummer and eventually drove them in. The window came down and I was able to remove the remaining nut and then remove the motor.

Bought the new motor at mercedesshop.com for $89.The motor wires needed soldering.

Everything is back in and working, sort of, but the motor is really struggling to get the window all the way up. Starts ok, but about half way up slows down and then barely makes it to the top. Tried adjusting the regulator using the only adjustable bolt but this had no effect. The window on the other side is twice as fast. I'd hate to close up without fixing this. Any suggestions would be appreciated