You can't get rosacea like a cold, nor can any irritation from a topical or procedure make you get it. It's a genetic condition and likely hereditary. It tends to occur more often in light-skinned people, and usually shows up in one's 30s. One can have early signs of rosacea, such as excessive or constant skin redness and flushing, happening in your 20s.
It is possible developing permanent hyperpigmentation with superficial chemical peels and scratching, but it's rare and occurs more often in darker-skinned people. If you had a botched medium-strength TCA peel, then I'd worry. If your scratches get infected, that increases the possibility.
Having truly severe acne makes it really difficult, if not impossible, to treat it topically. That's actually one of the criteria for severe acne diagnosis. One can have severe acne even if they have hundreds of blackheads and whiteheads!
How the pH relates to the skin is really interesting. Applying very acidic (2.5 pH, if I remember correctly) can make the skin sting. It does depend on the ingredient, too, like lactic acid and urea. pH bounces back pretty soon if you happen to apply something acidic or alkaline. Here's a really interesting paper about it: http://www.ncbi.nlm....les/PMC3175800/
The real skin damage occurs if it's in the dermal papillae or deeper. Having collagen and elastin not form properly or get cross-linked with something bad, for example, can screw up a lot of other things. Sometimes, gene mutations that involve natural moisturizing factors (NMF) causes or aggravates a lot of problems too, like with atopic dermatitis/eczema. Having severe acne can and does damage the collagen and elastin to the dermal level. That's why a lot of aggressive treatments like lasers and chemical peels target the dermal level--because the "foundation" is damaged.
Glad to know your allergies are better! Never had allergies myself, but I have family members who do. They really hate sticking to their diets.
You should probably go to someone who knows how to patch test properly, or really knows how to treat sensitive skin. You might have to get it officially diagnosed (weird, right?) before going on a treatment protocol.
Venting online is one of my favorite therapeutic past times, too.