for a start, there are different types of allergic responses, and different tests work in different ways and pick up different body chemical signals and reaction types- so its quite possible that the tests used with her aren't covering everything that her body is doing/reacting to, or picking it up in the ways that it does react- different body chemicals do different things- and most tests don't cover all the responses and chemical markers.
secondly, i'm pretty sure that if your daughters body hasnt come across an allergen- like dairy, for example- for a while, the levels of defense against it in her body, may have dropped, which might account for why it didn't show positive on the test. i don't show up as allergic to some stuff nowadays, because my body hasn't encountered it for a while, so its not making as many antibodies against it generally. i know though, that when it i eat it, (in a double blind test when i don't know its there) it still causes my body to panic- and i'll react.
i can't explain it properly, cos i'm not an allergy doctor, but i know from experience that the tests used aren't near to 100% accurate for near to 100% of allergies in near to 100% of cases. i don't show a response on the scratch test which looks for an igE response, to some things that by body clearly freaks out over (and was told i'm not allergic to it- try telling my hives that, lol) -this was perhaps 'cos my body's response was due to an IgG or IgA type response- (its confusing to me too) which the test doesn't cover and the doctor didn't recognise.
i think you might benefit from joining a free yahoo online group for parents of kids with allergies- i joined one a few years ago, just to look around, and they are a wealth of knowledge and support (and info on things like recipes and rotation diets). Additionally, i'd suggest borrowing a book or 4 which explains food allergies, from your library. There are some really good easy to understand ones about- have a look on amazon, and for some reveiws, then get your library to order you a few, and read read read.
for introducing stuff back into her diet- i wouldn't give her a big amount at a time. i'd be more inclined to give her a few drops, then wait and see for a few hours, then if nothing happened, give her a spoonful, then again, wait, and maybe try a 1/4 cup after a day or two. by ramping up like this, you have more of an opportunity to check for reactions without causing a big scary and uncomfortable one, and it makes it easier to see if she can tollerate something up to a set dose. she might be ok with as much dairy as there is ain a quarter bar of chocolate, once or twice a week, but not with a cupful every day, you know? depends on her body and her allergy type/sensitivity. the allergy books will explain how to do this too though.
good luck!
ETA: Learn as much as you can about this- allergies are crazy things- i'm allergic to grass pollen, and when my hayfever is really bad, tomatoes will make my lips burn- because tomatoes have got similar chemical properties to grass pollen. i'm not allergic to tomatoes though- i can eat them fine, as long as there is no grass pollen about for my body to panic over! its weird- but important to know how they work- saves worry and stress in the long run.