OK, firstly, that's scary about the feet - actually the owners sound really awful, so good on you for taking this poor girl on (and paying to)! My farrier went mad on me for leaving Brock's a week longer (i.e. 7 weeks, but his do grow quickly) because I got my dates mixed up. 3 months is crazy. I'd find a way of sorting that first, even just as a one off to see if it makes a difference to the bucking.
Regarding the bucking, when does she do it? Is it in reaction to an aid? Is it when she's doing something in particular? If it's after she's been ridden for a little while, it may be her way of communicating "OK, I want to finish now". Or it could be pain. Does she react when being saddled as well? Does she have any rubs, lumps or sores around the saddle area? Actually, you mentioned her saddle was too narrow - is she still in this one, or in another? Sorry for the thousand questions :)
My horse bucks when he doesn't want to do something, usually either because I've asked something that he doesn't understand (as in, when I stumble across one of his training holes) or when he has a bad memory of a spot and doesn't want to be there. Trick is to get them going forward-forward-forward. If a buck is imminent and forward isn't an option, or he starts backing up, then I turn him in tight circles. After a few of these, I 'open the front door' and he is happy to go forward. But obviously this only really works if the problem's mental and not physical.