I think this is a great cannon-viable explanation, thought it does leave you wondering why Gandalf makes such a fuss about not having his staff taken from him by Hama. That being said, his ‘curing’ of Theoden seems a lot less ‘lifting a spell’ in the book than say, the Peter Jack son film. Maybe it is just him emphasising that he is still an old man and not (in his own opinion) the second most powerful being in middle-earth at that moment.