I think it is oversimplifying the issues to assume that all Simmons needs to do is go to the gym and take 1,000 free throws. If the problem is more in his head than his hands, that isn't going to necessarily help. Simmons clearly has some maturity or emotional issues. To me, these are bigger than any physical issues, and harder to fix. That is the problem. Teams don't know what they are getting, a star player or a head case who will never fulfill his promise.
A year ago, these "head case" issues were not so apparent, at least not to the rest of the league. I would have been thrilled to have the Celtics trade for him and play him exclusively as a PF (no more PG). But now, I am not so sure. It may still just come down to restoring his confidence in a new setting and muscling through the free throw issues, I don't know. And not knowing is the problem.
I do feel that he has made the issues worse for himself though. He is making it really hard to have any kind of a good outcome for him.