Awwww poor you and poor poor little Lucy, this is so sad, regarding her 'sinking' tummy as long as she is eating and pooing, weeing I wouldn't worry too much, I would feed her up with lots of healthy hay and veggies and generally spoil her rotten, I actually got a lump in my throat when you described her not 'behaving' like most happy socialised, bunnies - that is just typical of all she could do probably, it just confirms the cruelty of locking them in a small box with no stimulation - it's like she has been 'conditioned' mentally to just sit there god help her
I hope your going to be able to have her indoors for a good while so she is really strong and has been rehabilitated before she takes on the other bunnies, I would house her next to them so she can watch and learn I think. Poor little innocent mite. I sincerely hope she recovers well and has a second chance at what life should be like. x