I am going to have to say that I highly doubt you are dealing with a Cocci problem at this age.
Is taking her to a vet an option?
Have you checked them over really good at night for mites?
Have they ever been wormed?
I know that Corid is not a medicine, but I have no idea if that much at one time would hurt or not. (I'd have to guess that it would not hurt

)
I think your best bet is to tube feed her, not eating will kill her.
I am too scared of syringe feeding/watering because I'm afraid it I would "drown" the chicken by getting it in the wrong "pipe".
If you search youtube for tube feeding chickens they are some great videos.