I think I should be able to solve the issues myself with experiments.
Hoping so anyway!
I upped the amount of fish oil a couple days ago so now she's getting about 1500 mg per day. She has some blemishes like in the very first photo in this thread, where the hair is raised on the sides of her shoulders. These blemishes come and go one week to the next, and I don't think there is a change in the blemishes' severity since I added in the fish oil. I do think that the irritation around her mouth has probably increased in the last few days.
Roxy will be finishing up this bag of Iams Small & Toy Breed in about 10 days, so in a week I will start switching her over to California Naturals Lamb & Rice. CN does not have any fish in it, so I can remove the fish (oil) from her diet easily if needed. I am not too keen on feeding a food from Natura, but she's on Iams right now, so who am I to be picky at this point?
I do wonder if Chicken might be the issue. Before I adopted her she was eating that homemade food (mostly white rice with some hamburger and carrots) along with the Iams, so that would have significantly lowered the amount of chicken she was consuming. When I switched her to Orijen and Acana that would have very significantly increased the amount of chicken she was consuming.