There is no need to speculate about what more might have been happening than what was said. For example consider his pot comment. We have known for many years

There is no need to speculate about what more might have been happening than what was said. For example consider his pot comment. We have known for many years that common herbicides, for example paraquat and maneb, can cause Parkinson's. Sometimes surprisingly quickly. In the 1980s we still used cyperquat which is even more closely related to https://en.wikipedia.org/wiki/MPTP https://en.wikipedia.org/wiki/MPTP which can (when injected) create almost instant Parkinson's. And not only did we use those herbicides, law enforcement specifically used them to target marijuana. So it is quite reasonable that smoking marijuana that had been sprayed with an herbicide could have caused him early onset Parkinson's. If the bad batch was smoked on set at Leo & Me, you've got the whole cluster explained. Or maybe they were unlucky and filmed directly downwind of someone spraying too much herbicide. There are a lot of possible ways that it could have happened.