As I say, these guys are professional historians, and they did a pretty good job rooting out the truth, so for them to say it's unclear says a lot. The book is great.
I'll check out the book. It looks interesting. I'm not a huge Wayne fan. I think that his persona worked well in westerns for a lot of reasons and not so well in other movies for the same reasons. In my opinion, The Shootist is one of the best "modern" westerns ever made. The Searchers is also a nice piece of work.