The Thornhill looks to be the right width, but I'm not crazy about the shape of the rear panel for her. It looks like it might dig in there/bridge in the middle with a rider's weight. I think, though, that you've got it positioned too far forward and it's interfering with her shoulder. When it's back far enough, I suspect it's going to be too long for her back.
I live in Sask, which is the far side of the back of beyond, Tack stores are few and far between, those of us who choose to ride English are a strange breed that are grudgingly given a corner, or preferably a back room, away from the *proper* tack.
I will have to get back on the local boards and see if there is someone about, last time I asked there was someone who knew of someone.......the nearest direct hit I got was 4 hours away!
I once asked a tack store in Tucson if they had any English saddles. The guy looked at me with an expression that indicated I probably engaged in bestiality, then led me to a back room that would have qualified as a saddle museum. I doubt there was a saddle in the room under 50 years, and most had gone at least 40 without being touched by oil. I was afraid to touch anything lest it crumble into dust...