A stubborn case then. I can't really add anything you don't already know, but yes, trying different stuff can help. Some use technical spirit, various spray can stuff are worth trying to speed things up. You might already have it on a shelf some where. On my 201 the bottom screw under the bobbin case (the one visible just under the bobbin) was completely stuck and what ever I tried it would not move. This machine was used regularly and there's an oil point in the center of it. Every time I oiled the machine it was given a drop of oil, I used Finish Line Ceramic Wet Lube, a light synthetic oil with teflon. About a year later or perhaps a bit more I made an attempt at the screw, and it came out with no effort at all.
I had a similar case with a three pin plug for another machine, it had be stuck for several years, but since the wire was fine just old I kept on using it. I remember I gave it a drop of Finish Line oil and three weeks later it was loose. I had tried different stuff in the past, which I think where WD-40 and regular sewing machine oil. There's nothing particular in the Finish Line oil for frozen parts, so I guess it's all about persistance and trying. Keep the machine in a heated room, it makes all the difference. Some of the spray can stuff contains wintergreen or methyl salicylate, it has proven very effective in more than one case.
Another thing that often gives result, is going over the machine a few times just doing the regular routine; oiling, further cleaning and inspecting all parts. The oil has had a chance to it's job and there's a good chance you detect something that wasn't obvious on the earlier attempts. It sounds odd, but taking a machine back in use, using it for only straight stitching if it works, moving levers, knobs, and tending to it regularly has sometimes worked better than anything else. I think it must be the attention and movement the parts get, oil seep even better into inners of stuck parts and all minor things will be tended to along the way.