If she has had a hard time and so underweight, I'm wondering if layers pellets could be too rich for her? Are the pellets and wheat already mixed or do you combine them?
I used to feed whole wheat in water, so they could enjoy dabbling for it, mine used to like weetabix, wholemeal bread, rolled barley? you aren't supposed to feed meat, but someone once told me they gave a little tinned dogmeat to help poorly ducks, (thinking of slug alternatives?)
good luck with her, she sounds like she's getting better already
just re-read you post, not sure about introducing her to 2 pairs, I think the females would gang up on her?