So I opened it up and gave it an inspection. The black splotches were caused by the deteriorating seal at the top of the film back next to the ISO dial on top (is that what you were referring to as a seal that isn't neccessary?). It is pilly and covered in ink for some reason. I found ink splotches on the main rollers as well as the little silver rollers which I didn't even know existed until now. I didn't have any cotton swaps or alcohol, so I used warm water and a paper towel instead.
Could the leak be caused by this deteriorating seal? The light leaks were horizontal, so I don't see how it could be the cause. Took off the film back, took out the dark slide, and tried to shine a light through the dark slide slot in a dark room and I couldn't find any leaks, though I did find more foam pills in the slot clogging it up.
Is the seal kit from Mr. Goodman for this foam specifically, or is there another seal I'm overlooking?