With my amazing $140 amazon-purchased borescope, I was able to find what looks to be the problem:
I'm pretty sure I can get the ProSeal back there with a syringe and some tubing. But I'm struggling to figure out how to spread it around. I need some way to manipulate this thing from 8" away through a 3" hole:
I could do it with a colonoscope, but I don't think I could get away with borrowing one!