Well I have read the entire thread and followed along. My concern is the OP’s need for a cheerleading squad and threat to flounce bar that.
The photos I have seen show the mare looking much better when she first got her than she does now. “Following the vet’s orders” is not the end all and be all of a refeeding program if the horse isn’t gaining and despite the cheerleading, it doesn’t appear that she is. There is blood work, parasite and teeth consideration as well as kidney and liver fuction, to determine why the horse isn’t gaining. Also, the amounts of feed she is getting may still not be enough for the horse.
I would think very carefully about putting a horse this thin on pasture for a midwestern winter, where she will be competing for her hay and feed. I am personally not a fan of feed bags because of the respiratory risks inhaling feed or moisture. I would never put a saddle on a horse with that backbone, and certainly would not put weight on it with a rider, no matter how many cheerleaders say differently. The horse could benefit from much of the suggested ground work but the OP doesn’t appear to be pursuing that and that is a concern.
I don’t think it helps anybody, the horse or the OP, to tell her the horse looks great and should be lightly ridden, when she doesn’t and shouldn’t.
I would get all the metabolic panels and bloodwork that would be applicable to establish her baseline health, and I would put her on a course of omeprazole assuming ulcers were present from her history of stress and I would make sure she always had high quality hay in front of her at all times.
I would keep the horse indoors at night and separate during meals, on her own personal feed plan that would mean she could eat in private and without angst, given her history of having been bullied off feed in the past. I would not ask her to brave the winter outdoors so she could sleep well and indoors at night, although being out during the day in her own private small turnout would be great for her as long as she wasn’t being bullied by other horses.
I would add a third feeding of whatever she’s getting to her routine for a mid-day meal. Horses are mean to eat frequently, and three, or more, meals a day is often crucial to getting weight back on a thin horse.
And I would work about 10 minutes a day right now, no more, on ground work, lateral work, backing, carrot stretches, stepping under herself, whatever is going to get her to use her back somewhat. Its been suggested but unfortunatey the OP hasn’t shown any photos of that work, if she’s started it.
These are the important basics the horse needs, and the OP needs to work on. Hopefully she’ll continute to post and work on these things. If the horse doesn’t start to gain weight, it is really important that the OP takes a hard look at why and work to find out what may be going on, and not let herself be petted into thinking she dooesn’t need to keep looking for solutions. Many things may be affecting her health right now, just, please, don’t put a saddle on the backbone that 's there and ride her. its not fair to the horse.