That's interesting about her going lame after a cold spell. How cold was the cold spell? Enough to stiffen the ground? That would add to the pain on her soles. If not, then the only other thing that I can think of is arthritis bothering her. How old is she? The MSM may be a bit hard on her, long term, but, no, the shark and gluc won't cause her to founder. I know that from experience. Just expensive! And I don't know if it really works or not, some people say it does, but It didn't for me with a friend's horse that boarded here and I cared for her for a year after she got off the operating table from a kicked hock that was broken in 3 places. The consistancy of her fluid was good each time the vet withdrew a sample. In her case it was the fact that the fluid has to wash into and on out of the joint. It was coming in o.k., but couldn't drain fast enough and would build up and puff out the joint pockets causing tension and pain. As soon as the vet drained it, it was like a miracle, cause she was suddenly sound and pain-free. A week later, it would build up again though and we were right back to square one again. So the glocosamine didn't help us.
The experts have decided that one of the best treatments for founder is to put them up on heeled shoes to take the strain off the sole and the back of the foot. I'm surprised that the vet didn't tell you to call the farrier in to do this, right at the beginning. They've sure found that it helps with the pain.
Maybe another assessment is in order, another set of xrays and see if raising the heels will help at this point or not. If she is in pain from founder, then I believe it will help. If you are finding her lame during cold spells, I would skip the blanket and let her winter hair do its job. It will do the job quite well. (as long as her coat is dry) I would bandage her legs instead. Nice big puffy-type sheets under elastic-type wraps. Keeps out moisture, cold, promotes blood flow down the leg and keeps them toasty warm too. It will also help a lot with supporting her legs. Remember, if a horse is favouring one leg, then bandage it and also the leg beside it,cause you can be sure that if she is favouring the bad leg, then the good one is going to take a beating. Gotta keep the good leg good so it can help with the bad one. Good luck.