I honestly think she looks fine. She doesn't look underweight to me. She has her winter fuzzies but is pretty shiny for that, and nothing is jumping out to me that she's underweight.
As well, I would question who said she was selenium deficient, and what they used to test for that. Was it the vet who did a blood test? Or was it some random who maybe had an "experience" a horse wht selenium problems, and figured yours did too?
Maybe it isn't her diet that is making her under-muscled? The type of work effects muscle development. You can feed her right, but she's not going to get muscle if she's not worked to develop it in the right places, if that makes sense.
If there's that much confusion at your barn between you and your BO, then I suggest you and she sit down and talk about the problems you have. Then the air is clear and confusion is no more.