is she drinking......i know you want her to eat, but i would be very concerned if she wasnt drinking either..
a growl in my opinion is a good thing as at least you know when something is hurting her and know when to stop, a muzzle shouldnt be too much of a problem unless she associates this with pain or discomfort, then she could become difficult.
try not to panic too much (easier said than done i know because i would be a mess by now) but she will pick up on your behaviour.
perhaps some fish sandwich paste licked off your finger, as if she has a sore mouth or throat she doesnt have to chew this.....and of course, it stinks and we all know that cockers loooove stinky things..
please keep us posted, ....lyla has really tugged at my heartstrings all over christmas, and i would love to hear of her making any progress, however small.....