I don't think he needed to cocaine to put a strain on his sanity, he was pretty much away with the fairies before all that started. 'All the Madmen' came long before 76 and as far as i'm concerned only a first class fruit loop could dream up a song like 'Rubber Band'.
He's by no means a "cocaine cripple", that would imply all sorts of unpleasant characteristics like being totally unable to articulate yourself (look at Brian Wilson of the Beach Boys for a good example of that).
I think he has a love/hate relationship with the issue of insanity. He probably likes the eccrentic englishman role but of course is petrified by the legacy left to him by his brother and other relatives.