Well, if she was just recently caught when you adopted her, her system is likely still trying to adjust from her sparse diet in the wild, vs a domestic horse diet. Also, assulting her with massive amounts of dewormers isn't going to help at this point. A regular deworming routine is fine, but with a diet, exercise change, and water source has changed, not to mention just the overall stress she's going through is enough to cause problems. Esp if you consider the BLM pretty much just feeds Alflalfa hay, so that was a shock, now you are feeding your own stuff, another change...
Right now, I'd not worry about a hay belly. As long as she's looking otherwise healthy. Mustangs do tend to have a different look than domestic horses. I had a couple I adopted a few years ago. One was mature, one was a yearling. The yearling looked frumpy for over a year, before he really looked pretty, and he was healthy, it was just his awkward growth that made him look funny. Big head, big belly.
Deworming on a 2-3 month schedule is more than adequate. When the BLM gets them, they work them over pretty good, with shots and worming, and overowrming can cause be harmful. Remember, if she had worms before her system has some immunity to them, and massive die-offs of worms can cause colic on its own.
It's quite possible she has an ulcer from the changes of diet, the chemicals and stress. Just give it time, try not to change anything eles, except to drop the oats from the diet.
I would omit the oats-if she's underweight maybe she needs them, but really, grass hay and beet pulp with a mineral is plenty. Grain should be a supplement to hay if they are struggling to keep weight on with hay alone.Remember, horses are hay burners, and aren't meant to be fat. As long as you can still feel the ribs, but they aren't highly visible, you are in a good weight range. The round, full belly is more comsmetic than anything. It will tighten up with exercise.Feeding her a bunch of grain to make her grow is a bad idea as well. Another point worth mentioning is that the oats translate to sugar to your horses's system-something I doubt she's ever had before and that can take a while for the gut to adjust to-causing diarrhea.