Apparently, William Dettloff’s book Ezzard Charles: A Boxing Life documents that Charles first felt weakness in his limbs, and some numbness, as early as 1951. This was 3 years before the Marciano fights, and it was also the same year he lost his title. Is this true? And if so, does it take anything away from Rocky and Walcott's wins over him?
It's certainly possible that he was in the early stages of the disease. After fighting Rocky he was a sub .500 fighter to finish his career. Now that could have been because he was an old fighter with a lot of wear and tear but it is certainly probable that he was already sick. I think most would agree that Charles was the superior fighter to both Walcott and Marciano. He simply was past his best, possibly sick and fighting above his natural weight against those men.
I don't know what the book's source is, but in this interview from 1968 he says that he first noticed it in 1955. Mind you, Charles wasn't always consistent in his recollection of dates, as anyone who's tried to pin down his sparring session with Sonny Liston will be aware. https://ibb.co/bKmDnVB
It's a possibility, although I would doubt it personally. I think it may have been before the Louis fight (memory is really hazy here) but Charles had a medical check due to concern about a heart problem (I think, may have been something else) and they found nothing adverse whatsoever. I don't think in the few short years between that and Marciano anything substantial would have developed. I believe the accumulation of fights prior to Rocky, combined with the brutality of their fights culminated in his condition. I'd hazard a guess and say that Charles didn't have it before Marciano but definitely did after. As for symptoms, if he had any, they weren't notable to be written in his autobiography.