Just to update this; I followed the suggestions above and managed to get it rolling up and down. The regulator was not frozen and once I got the weaterstripping free, the whole thing started moving.
It was NOT a quick job, took a lot of patience and gentle tapping here and there, plus using a bondo spreader and gasket scraper here and there to work the penetrating oil in.
I do have a new channel kit and weatherstripping coming.
The drivers' side, unfortunately, the regulator is already broken. I went to repeat over there (there was no handle on the crank, should have tipped me off) and discovered the splines are stripped. When I went to try some pliers on the shaft, the shaft actually just spins freely. So, one regulator to add to the cart on Monday. That's all right, could be worse!
Last edited by NorthCoast3800; 04/05/2020 3:50 AM.