If I was in your shoes at this point, I think I might take the camera into a darkroom and cut about a five inch piece of film off a roll. Lay the film across the normal film path with the left end tucked toward the front around the take-up spindle. Then let the camera sit in bright outdoor light for five or ten minutes. After that, go back to the darkroom, remove the film and develop it. By not moving the film at all while it's in the camera, you should be able to see where the light is coming from. You can get four or five test strips out of one 20 exp roll for multiple tests.
A couple of your ten frames show apparently different effects, such as a streak or multiple "stepped" exposures. That's even stranger yet, but knowing exactly in what portion of the camera the light hits might provoke a eureka moment for the rest of the questions.
Alternatively, following JBrunner's lead, if you could manipulate some light to get a shape resembling the "Blessed Virgin," you could flood *bay and retire.
Pick up another "Brick" and pitch the one you have
Aw, come on now, where's your scientific curiosity?
We might be on the verge of discovering a time related degradation in this entire class of cameras! And it's possible it could be fixed by a tiny dab of black goo. I've had mine for nearly fifty years, since new. If it was acting up, I'd want to try fixing it.
(Truth be told, if my C3 expired I would let it be a sleeping brick, as I have three Canon SLR bodies and an assortment of FD lenses for my 35mm needs.)