I've got a couple of other things you might consider.
You'd know if the lab was having a light leak if you just ask them. If that was the case you probably wouldn't be the only one asking about it. Meantime, send a roll to another lab and double check.
Film tensioning at the end of the roll and leaking around the last frame? I doubt that. 120 film has a paper backling throughout the roll unlike 220 film that only has it on the front end and the back end.
Seems that if you had a tensioning problem you'd likely have flatness/focus issues on the rest of the roll that would get worse towards the final frames as the take-up spool collected more film and it was so loose that the pressure plate couldn't keep it flat. I think that light leaks from failing to tension up the film after downloading produce far less consistent results then what you have here.
Finally, try unloading the camera in a darkroom or light tent. If the finished negs/slides show light leak, then it ain't your technique but that would point me to the camera and how snug the film is being wound. Seems like you've covered all the other bases except one:
As the take-up spool thickens towards the end of the roll, is it twisting the take-up and pushing against the camera back? Check the spool alignment. You can take a test roll and watch it run through the camera paying close attention to what happens to the spool as it gathers up the last frames of the film. That might be a worn gearing problem.
Take it errrrrrrrrrrr. "light"
Mark.
Mark