The first thing I'd try was to replace the key. I had one of those re-introduced Colts once, that I think were made by Uberti. As I pushed in the key further it would actually jam the cylinder face to the breech end of the barrel so hard that it would not turn. When I backed it out a bit the problem was fixed.
So you might just try a new key. Anyway it's a try. All of the other solutions here look okay too, but I'd try a new key before I went to all of the work.
Say, someone really disrespected that revolver didn't they!