I think I'm out of my depth here Rickman, but I remember a homeopath making a lot of fuss about why I should avoid parafin. She recommended a cream (I'll try to find it, I'm not sure about the name) that worked well but considering it came in small tubes it was too expensive for the amount we used per day.
In terms of testing... I have not come accross with a test for specific E numbers yet. I don't know how long it will take for a dermatologist to see her but, if she is OK with Aveeno so far, what may use it as necessary (several times a day I guess) for about 2 weeks, then try again the Diprobase in a small area and see what happens.
Is she still taking the antibiotics?