Good job James, I hope the film is good, I'm going to shoot a roll of 616 in the next week so I will have a few spools with backing paper. I've seen similar slitter designs but they had a narrower gap to feed the film through, just enough to easily slide the film in. That may solve your problem getting it started. The film may be curling over rather than starting the cut.
I have a few slitters for dividing Double 8mm film after its processed. They use offset rollers, one pushes the film up and the other pushes it down splitting it in the middle. They are easy to get the film started and work quite well. That design would be quite difficult to duplicate without access to precision machining tools.