Right, a cable to go to the light, that makes perfect sense. I'm very much looking forward to learning what your mechanic finds out. Fingers crossed that it's something minor
One final thought, on a model 15 I worked on some time ago the feed dog lowering screw was completely frozen. I'm wondering whether if there's an issue with that mechanism, it might freeze the machine. To lower the feed dogs, one loosens the screw and flips down a lever. Yours looks like it has a fair bit of oil varnish on it. Does that mechanism move properly?