Advice needed!!!

Hi new chicken enthusiast here with a big issue! So I have an injured 13 week old, barred rock hen. Let me first say I do have a vet apt scheduled this afternoon. Introductions into my existing flock ( 6 hens, 6 months old) did not go over so well, on my second attempt. They have been living side by side for weeks. I have followed all the guidelines and still on my second attempt by day 2, i wound up with a hen whos head/neck area pecked. I of course removed her and her buddy placed both of them in my garage in seperate dog crates, with free time outside during the day. My injured hen was treated w blu kote initially hoping to keep her buddy from pecking her, but to be safe i seperated them anyway. Im on day 3 now, this little hen keeps me feeling the ups and downs. One second shes eating a little ( yogurt, bananas, scrambled eggs) and drinking a good amount ( just added honey to the water) but next second her posture scares me: she can no longer stand easily, eyes closed but alert when need be, a tad slumped with feathers out. She has passed some greenish poo which from what I read is lack of nourishment. Is there any coming back from this? Will an antibiotic really work wonders or has too much time passed? I want to be humane and do whats right, she just keeps throwing me through the loop and remaining hopeful, but my gut tells me to end her pain. Shes so young so I didnt want to give up on her! Please any input and experiences appreciated!