I am going to share a bit of my own story with you as there are a few things you have written as well as the appearance of your bird. I have marek's in my flock. If I saw this, I would sigh and wait for the inevitable as I have seen this too many times to count anymore. The runt issue gets me first and foremost. Usually the runt has come from a bird that has problems whether it was known or not. While this age bird shouldn't just being showing symptoms if it were Marek's but it is possible. But! It is also possible that it is something else that is causing her problems. Parasites, internal or external can do this. They will suck the life right out of her if they are not gotten rid of. I'd really like you to check a bit more for something else besides Marek's. If that's what this is, your whole flock has it and for all intensive purposes, so do you or at least you have to think that way. You will not be able to sell, give away any birds you now have, nor will you be able to bring in new ones without following the proper steps and they are hard to follow for most. You shouldn't go and visit anyone else who is keeping poultry nor should they come to you. No 4-H fairs, no animal shows of any kind. Marek's is serious and pretty much final if you can't take the steps needed to breed what you have for resistance. This is why I say, look for the other things first. Do you have an avian vet near you? I would start there. They can do the blood work and tell you what it is.Ok- i just started giving her about 500 calcium with 500 D as of last night. As far as vitamins go, here in Philadelphia I don't have much access to bird vits and supplies unless you can buy it at Pet Smart.
Where are you buying her pellets? Our Pet Smart stores do not carry chicken feed of any kind. You can order the vitamins online from someplace like Jeffers but there really should be someplace fairly close to you to get them there. You really don't have a Tractor Supply store nearby? No feed and grain stores?
You said you'd give her something other than fruit- how about Pasta? What would you give her? I stopped giving her pellets because 1) her crop was a solid lump for a day and a half, and it nearly killed her. 2) it seems to pain her to eat solid foods, and I want to keep her hydrated, and I know she loves grapes, so... grapes. Should I give her starter feed?
I would put here back on a good layer pellet or crumble and put some ACV into her water. 1 tbls. per gallon and alternate that with the vitamins and electrolites. She has to eat and grapes and such only go so far. She needs a balanced diet that the chicken feed provides. If you're concerned that it may be too hard for her mix a little warm water into it for a day or two and see what happens. You only want to mix small batches of this, as it can and will go rancid and that would cause it's own issues
She weighs in at 2lbs 12oz, but she was a runt from the beginning. I know she's lost weight over the last few days, but I'm sure that she hadn't been more than 4lbs on her best day. Her breast bone sticks out a bit because, I think, she is not eating much of anything. Not for a lack of appetite though. I just don't want her little crop to get jammed up again. No mites, I checked.
When was she wormed last?
She does not stand- her legs seize in a flexed position- she moves them and is trying to walk though. When I set her down, I try to see if she can stay/balance upright and centered, but she always turns off to her right side. I have been flipping her, L to R, for fear of the unknown.
This started on Saturday morning (perhaps Fri night). I found her hock-sitting in the secondary coop that's low to the ground (she has always favored this one). Her toes were balled up into fists, and her face was red. She had plucked the feathers off of her behind. I took her in, sat her in a warm bath, and then oiled her vent. She passed a bloody egg a few hours later. She could stand, she could walk, but her legs were, again, hyper flexed. In other words, she looked like she was walking on stilts.
Have you checked to see if there are more there? Has she laid any other eggs? From just between her legs gently feel towards her back end and see if there are any lumps. It might not hurt to give her the calcium for a few days.
Excited that the worst had passed, I tucked her in to the crate and went to bed, sure that she'd be OK in the morning. She was on her left side, and had plucked a bunch of feathers out of her chest. So I set her up in the "sick bin" and bought pedialite and watched her. Her face was BRIGHT red all the time. She was a tad lethargic.
A half a day later I noticed the lump in her crop, and began to give her 1-2 grapes served in a little dish of olive oil. She ate and drank with zest. The lump passed on Monday morning, as evidenced by the foul-smelling rotten poo that she delivered. Since then she's been pooping wet but on her own. She went on a strictly no-pellet diet on Sunday. She started cipro in 15ml OJ at <.5ml per dose on Monday. SHe also started getting the calcium/D since Monday night...
Hope this short story helps...