Hi nicchick,
Can you tell if she is laying or has laid recently? Have her eggs looked normal to you?
it is odd to have one very fat ex bat as they share the same diet. So with this in mind might be worth checking there is nothing else wrong with her. Does her abdomen feel squashy? (have a feel underneath between legs and move hands gently forward towards her chest). You mentioned she was unsteady -is she slightly breathless? Look at her beak is it opening and closing frequently?
Have a lot at her dropping as well. Do they look normal. or watery or maybe yellowy almost like scambled egg?
I am wondering that she may have laid an egg internally.
In the meantime just feed exclusively good quality layers pellets/mash.
Hope this helps and keep us posted on her symptoms