Boy, that is a pickle to be in - allergic to soy and alfalfa both
Do you have access to an equine nutritionist that is NOT affliliated with any sort of feed or supplement product? And that includes not asking anyone from SmartPak.
If you have a university in your state (hopefully on your side of the state so they are familiar with your soils, allergies, etc.) that has an equine progam, my best suggestion is to call and ask to speak to an equine nutritionist.
You can buy many things to get fat on the horse, but your dilemma is protein and amino acids that the horse needs to maintain muscle health.
I do use timothy/alfalfa cubes on my 24 yr old that has metabolic issues and can't have soy. I feed him EquiPride vit/min supplement because it is soy-free. He suffered a lot of muscle mass loss when the EMS exploded on him, but adding the tim/alfalfa cubes did an amazing job of building him up again.
Point-being, hopefully an accredited equine nutritionist can help you formulate a program that will support your horse's muscle health
EDITED TO ADD: Hopefully she's not allergic to Vitamin E because I would start pumping her full of it. I have read a few articles that say it's ok to feed 10,000 IU daily as a maximum, but I think they are being conservative.
I feed the EMS horse I mentioned 12,000 IU daily during the summer months and cut him back to around 3,000 or 4,000 IU daily during the winter. I never jump him from 4,000 to 12,000 but increase the amount gradually.
Vitamin E is a great immune booster.
Vitamin D is also a great immune booster. There was a lady on another forum that fed her Mule massive doses of Vitamin D because the mule's allergies were so bad she was practically hairless. BUT is was also under a Vet's directions because Vitamin D can be toxic.