URGENT help with sick bird

I brought her to the vet already but want to make sure I'm doing everything possible. She's ~9-10 weeks and we've had her for a little over a month. She was great all day yesterday, I was outside with them most of the day. Let her out at 7:30 this morning and all was well, then when I checked on her during a work break at 10 AM (I work from home) she wasn't with the flock and was very puffed up, standing with her eyes closed. I picked her up and she just melted in my arms, and she is a snuggly chick but usually resists being picked up. They free roam for 5-8 hours/day but we are usually out to more or less supervise, we don't use anything toxic at all in the garden or elsewhere. None of the other 5 chickens she coops with are showing symptoms. She had green diarrhea at the vet and the fecal was negative for bacteria or parasites, but it may have been too small a sample. They sent me home with antibiotics and I've been syringe feed and watering her for a few hours now. This morning she did eat some meal worms but wasn't interested in food, and I got her to drink some ACV water on her own but now she's only drinking through syringe. She is now very lethargic and sleeping in my daughter's arms. I have her drinking ACV/electrolyte/probiotic water through a syringe and eating mushed up crumble with the same water mix and a tiny bit of liquid vitamin A through a syringe every 1-2 hours for now. She is inside and warm.

We have 5 other chickens and a turkey outside and 4 chicks and 3 poults inside so I'm trying to prevent anything spreading and have quarantined her and will wear different clothes to care for her. The green poop indicates that she has been off food for a while, but I saw her eating throughout the day yesterday, hunting and catching bugs, and she went wild for mealworms several times so I'm stumped. As far as I know they are not vaccinated for Marek's. I'm worried that this sounds like Newcastle disease, and we picked up the 4 new chicks and 3 poults this weekend. The new birds have never interacted with the other chickens and older turkey, and they are kept inside. But we did wear the same clothes to interact with both flocks. Could that have caused exposure? And has anyone had a bird come back from something similar?