She has been wormed twice since her escape from hell. I will probably worm her again in a week or two.
First, I would not do that to this horse unless the vet recommends it. Excessive worming is the first thing that makes everyone feel good about a rescue. They need wormed but not that much - it can lead to stomach issues plus if the horse was polluted with worms, over-worming can cause the worms to literally eat right thru the stomach lining.
She's been wormed twice, she should be good until the end of January or February since you live in a cold climate. Taking a fecal sample to the vet before assuming she should be wormed again would be the right thing to do My two cents regarding weight gain is to let you know Tennessee Walkers are on the predisposed list for metabolic issues. Two of my three have insulin issues.
Point-being please don't pump this horse full of oats/corn/soy and corn oil, just in case. Too much starch in the grains, and corn oil is high in Omega-6, which is known to exacerbate inflammation. She may not have been metabolic before but the stress of being starved to death could trigger them and she sure is at the right age.
Not knowing one iota of her health history, I would approach her weight-gain diet very conservatively. Even though ration balancers do have grain in them, they have vitamins & minerals in them and would still be safe unless she does take a definite turn down the insulin road by going into founder.
Feed her a clean high quality mixed grass hay - hopefully something that is grown in your region. Until I knew more about how she ticks, I would avoid a lot of alfalfa; maybe just sprinkle a bit on top of her grass hay if you think you need to feed it