I am sorry that you lost her. You could do a home necropsy on her, opening her belly to get a look at her abdominal organs. That is what I do when I lose a hen. Taking pictures of the liver, intestines and other major organs and abdominal contents can show cancer, ascites or water belly—yellow...
Yes, that is the right SafeGuard, dosage 0.25 ml per pound. Pea-size is not an accurate measure. If depends on what type and strength of ivermectin as to the dosage. For instance the 5 mg per ml pour-on ivermectin dosage is 0.1 ml per every 2 pounds of weight.
Worming her is a start. It sounds like she has had a hard molt, and now her comb has that dry pale look they have when not in egg production. That can be normal in winter with short light days. I would give her a bit of chopped liver, tuna, and alternate with scrambled egg bits, but mainly her...