No, I don't know why colds cause flares. I guess the body is so busy dealing with the virus that it can't put its attention into preventing irritants getting into the skin.
I like the idea of being an Allergy Peer Supporter, although I'm not sure I know that much about allergies (yet!). My own allergies are less bothersome (penicillin, cats, hayfever).
DS has only had blood tests for nuts, after DS had a skin reaction (hives) a couple of times after eating hazelnut butter. I've done food exclusion diets for dairy, soya, eggs, gluten and nuts (the last of which didn't seem to make that much difference, but I think I must have picked a nut he wasn't allergic to!) I think it's hard to tell - he has enjoyed eating almonds, which apparently he's slightly allergic to, and I'm pretty sure he's eaten peanut butter which he is now definitely allergic to. 
The GP has asked the dermatologist for skin tests for nuts too - I don't know whether I can suggest they test for other things just in case.
Has your DD been reacting to only small amounts of wheat since you reduced/eliminated it from her diet? If so, that suggests it could be a problem for her - when you're eating lots of something it can cause fewer symptoms than when you stop & re-start it again. e.g. when I ate dairy, I was generally rather tired but had no other symptoms, but after stopping it for a fortnight, I got diarrhoea every time I ate anything with lactose in.
That dry scaliness after the infection sounds familiar. I found that was horrible - the patches scabbed over and then cracked. It was really sore for poor DS - his eczema is mainly on his face. We had a couple of nights where he woke 15+ times because it was bothering him (hooray for co-sleeping!).
I found that applying emollients as often as I could during the day and every time he woke at night soon improved it. Creams are better than ointments when there's an infection or weeping eczema. The only thing DS could tolerate on his face when it was like that was Dermol, which normally we only use for washing.
Apart from the Elocon cream, we have Protopic to use when we don't need steroids. It's an immune-suppressant which stops the skin getting so irritated. You can't use it if there's an infection (since it stops the skin fighting back) but otherwise it's pretty good. Some people find they don't need steroids if they use Protopic. We still do (often!), but it does make a difference, and helps to avoid the 'rebound' effect when you stop using steroids.
button, thanks - Sainsbury's is my local supermarket so that's good. :)