I would never have a vet do my horse's teeth. Vets spend about 1 or 2 days "learning" how to "float teeth." They may be able to identify pathology, but they don't know how to properly fix the problem.
I would suggest calling Spencer LaFlure who is an actual equine dentist, and a fine one at that! Or, I can PM you the name of the guy we use who lives in Wisconsin, and he is one of Spencer's Instructors. They have a different approach to floating teeth, and no one else will ever float my horse's teeth! I love the work that they do, and so do our horses.
And I would NEVER, EVER, allow any dentist to but a bit seat in my horse's mouth. It does not help the horse become more comfortable. This is what our dentist had to say about bit seats. I quote him word for word. "If you need a bit seat to make your horse comfortable, you don't need to be riding. Also, when you put a bit seat in, you take away one molar in the horse's mouth. Horses have had those teeth in their mouths for millions of years, and all of a sudden they don't need it?? It's just rediculous and it really screws up your horse's mouth."
The weight loss and dropping food could certainly be as a result of her teeth. So could the bit grinding. Normally that kind of behavior is associated with pain (which might be likely b/c her TMJ might be off), boredom, frustration, etc. Maybe she needs more tounge relief from the bit. Some horses gets very frustrated with a bit that goes off of mostly tounge pressure.