Pullet can not stand, drinks but does not eat.

So, my friend found this chicken abandoned in a lot about a little over a month ago and I ended up rescuing her and taking her home. When I brought her home I figured out that she was a sussex hen, she was still peeping a bit and didn't and still hasn't fully developed feathers. Her toes were badly bruised and swollen, her legs looked 3 sizes too big for her body and she walked pigeon toed. She also ate excessively as if she was never going to be fed again for the past month-ish. Her feet eventually recovered but she was never very good at walking. fast forward to this week. I went away for the weekend and left her unattended with my other hens who are very self sufficient and when I returned she was sitting outside in the chicken run not moving. I picked her up to put her in the chicken coup but took her inside because I realized she couldn't stand.

I'm not sure how long she was out there and I have done tons of research but am still not completely sure of what is wrong with her. I narrowed it down to possibly Marek's Disease, a vitamin deficiency, Mold? Maybe a slipped disk? I don't know this chickens history because I have only had her about 7 weeks.

She doesn't have the splayed legs of Marek's that I've read about, she was and is sitting with her legs curled under her. It appears that she has frostbite on a couple of her toes but I'm unsure if this is from when I originally found her (it was cold when I found her abandoned) or this weekend.

She is drinking a lot and alert other than the fact that she can't stand. Her wings sort of move, but not much. She doesn't have any interest in eating. I have tried giving her olive oil with a syringe (which is why she looks greasy in the pictures), b2, unfiltered apple cider vinegar, yogurt and honey all mixed into her water since that's what she's only interested in. I've tried blending her food into liquid so she can drink it so she is at least getting something. I have given her epson salt baths everyday but I'm not sure what else I can do for her. So far she's been like this since Sunday afternoon so only a couple days, she hasn't improved or gotten worse but I don't think it's very pleasant laying down all the time or eating. I'm thinking if she doesn't improve by the weekend I need to think about putting her down.

Also of note, I feed my chickens organic layer mash, chicken scraps, and their bedding is hay and gets changed weekly. If anyone has any advice or thoughts I would greatly appreciate it. I would like this chicken to not have to suffer since it seems that's how her life has been so far.