> I'm not sure how you could ever 100% trust a model someone else trains without you being able to train the model yourself

Are sure you can even trust the models you train yourself? It's possible that a model you trained is defective in a way you don't realize, e.g. will not recognize speeding cars with a sticker that has a picture of a bicycle[1]. It's likely that someone will discover that vulnerability before the publisher does, given the current state of ML model observability; ML model exploits are going to be wild, and inexplicable.

1. or the letters "GTEHQ"