Those excoriations are quite severe, and she certainly looks miserable, I am so glad that you are going to catch her and take her to a veterinarian, she is sorely need of care. Potential causes for the itchy
rash include skin
parasites (demodex, ectopic ear mites, notoedric mange), fungal skin infection (ie. ringworm, usually causes scaly hairloss and not as itchy as the pictures look), and food allergy. With the pus you describe, a secondary bacterial skin infection is likely.