I'd like to try and reintroduce slowly to the flock, just to see. If they act aggressively, then she won't be able to rejoin. If they accept her without too many issues, then I know she'll be able to be around them outside and learn how to be a chicken.
The vet did give her a clean bill of health aside the wry neck. She examined her and said it was a vitamin deficiency, and her stool sample came back negative for anything else (which means the occasionally red stuff is either stress or intestinal shedding).
Just leave the grit available free choice - they don't have to have grit mixed with feed.very small amount of Chick Grit (1x/day, usually lunch, sometimes every 2 days to be safe)
One of the better ways to re-integrate would be to house her right next to the flock so they can all see another all day long. Depending on the temperament and breed(s) of the flock, she may be picked on - that would be normal, but "should" lessen as time goes by.
It's good the vet examined her. A disease like Marek's can't be ruled in/out upon physical examination - only through necropsy.