I've developed the test roll, and it was a disaster.
Industar after cleaning: much worse than before, far away from being sharp. This lens is a part donor at best.
Soviet film canister doesn't leak light, which is great. But it doesn't help in terms of keeping the film in the right vertical position, which is what I hoped for. The film moves through the slit with considerable friction.
Zorki 4 simply slows down in the cold. I don't expect anything with this guy. However, Zorki C got some serious banding in above freezing, half of my test roll is a complete trash. Time for a CLA (at least I'll take a look under the top plate, I think). I've tensioned the springs as a temporary solution, tossed the camera into a freezer, took it out, and finally I've been able to pin-point exactly at what moment the issue starts: when the camera is cold, the first curtain stops somewhere along the way and both curtains are held like that, released when I release the shutter button. Doing photos in the wintertime with a camera like that isn't a good idea. It would work on B time only. Not sure if I want to have fun with these cameras at all or not, for now it's been a waste of time, at least in the cold.
Bad and ugly day today is what it is. Hope yours is/was better.