how would I test the acidity level of the cheese itself.
Either directly with a pH probe, or by taking a small sample of cheese, pureeing it with an emulsifier and distilled water, and measuring the pH of the puree.
What happens if the humidity is not maintained during those 2 or 3 days, which is what I'll be going by, but with an eye to actually testing acidity in the future?
Are you confusing humidity and acidity? Your sentence has both in one place, I'm not following. If humidity is too high in the first 2 days, you will get excess surface growth of yeast and other flora, even though there's salt on the surface. It will also not "seize up", meaning not firm up as well, which is pretty crucial. If too low, it may crack or cause moisture gradients in the cheese.
I've salted the cheese with 1.80 ounces of pickling salt, about 3.5% of the cheese weight as directed, but it doesn't really seem to want to take that much salt.
Correct, there will be a bunch of salt on the surface. It's fine, that's the point. You build up a salt gradient with something like 10% salt at the surface. The middle is not salty right away, which gives the blue a chance to grow and keeps the rind white and without blue. It's a classic approach, used for the traditional roquefort and other blues.