Other places to check:

The negative carrier/condenser at the photo lab. I've seen some real problems go unchecked at some places.

The other is the shutter gate. There might be dust or debris clinging to this part of the camera that hangs down in front of the film. I once sold a Canon AE-1 Program and the customer came back from vacation very angry about a thin white near vertical line through every shot. I opened the camera, set it on bulb, opened the shutter, and found a very thin, almost invisible thread of black enamel paint that was the culprit, right across the lens side of the shutter gate. The problem was solved in three seconds, but it didn't improve the existing photos. The customer was even more frustrated after seeing how simple the fix was.

Perhaps look for something hanging down between the lens and mirror box, just behind the lens mount, maybe only in certain orientations. Could be some deteriorated mirror foam there that could affect both finder and image, but it might fall out of the way when you take the lens off and hold it in certain orientations. It could even be hanging from the bottom of the inside of the mirror.

Of course not all of these should show in the finder, but might be in the same area as a finder problem by coincidence.

Lee