Opinions only, I know how to ride.....?

Well I am going to disagree with your fiancee on this one as an owner of Arabian horses our horses are "broke" in bosals and don't even go in a bit until they are five or six. They show all the way to Nationals most of our Junior Western Pleasure horses in bosals. These horses are showing at the big time on the lightest of rein pressure, under lights, music and crowd cheering, they seem pretty "dead broke" to me. My motto is always use what is most comfortable for the horse. Your horse obviously went better without a bit so kudos to you for figuring out that. Obviously there is still some problem with his mouth either real or residual from his previous incident but using a bit seems to make him more uh whats the word unbroke??? They make a bitless bridle now, most people don't even know there isn't a bit there so you might try that.