Ahh gees Ive delivered a mess of those!
haha. Dude, this is the highlight of my career so far. The little (if you can call 150 lbs little) bugger is CUTE!!.
As far as your questions go: I personally don't put as much stock into the blood tests as intradermal testing, but like i posted before, you need a dermatologist to do the intradermal testing and do it correctly. I'd look at the blood tests as "guidelines" not a be all end all determination of what your dog is allergic too. The real problem is with blood tests you are not measuring an actual allergic reaction (as you do with the skin tests) you are measuring a concentration of antibody circulating with in the blood and then interpreting it as causing the outward signs. Obviously there is potential for loss due to misinterpretation or you can have an animal that had an immune response but didn't manifest outward signs.
You can keep an animal on hydroxazine long term with minimal side effects. Its the same as a human using long term antihistamines. The one thing you need to consider is that just like antihistamines in humans, the dose given to a dog to effectively combat allergies may change with time---ie while 1 tablet is enough this year, in 2-3 years you may be giving 1 1/2 tablets. Becuase of this, if there are times of the year where your dog isn't having allergy issues, consider cycling off the antihistamine. It'll prevent/slow down, having to increase the dose at a later time.
As far as frontline goes---its also a very safe drug for topical use on all species of animals EXCEPT domestic rabbits--they have issues with it. I have never seen a generalized reaction from frontline, but I have seen localized reactions in some species, particularly cats. Typically its an area of redness, scratching, and maybe hair loss. They seemed to be self limiting in how they acted. Frontline is one of those drugs that if you dont' have a flea or tick problem, I personally wouldn't be using it---its just too darned expensive. If you dog has allergies to fleas, you may want to use it as a preventative, but I'd only do that if you lived in an area with a high likelyhood of flea infestation.