RA is a mathematical concept, not a tangible "thing". Negative is fine, and often desirable.
Alkalinity of 300 before or after dilution? Acid works, but won't affect the other ions that are most likely also very high. In fact it adds more. Most people don't like to hear it, but diluting to around 50 ppm alkalinity will really alleviate a lot of your headaches.
There is nothing wrong with adding acid to neutralize alkalinity. Your only problem will be finding your taste threshold for the lactate ion. Each acid we use in brewing will dissociate into a proton(s) to neutralize the alkalinity, and the anion of the acid. Phosphoric (phosphate), sulfuric (sulfate), hydrochloric (chloride), or lactic (lactate). Phosphoric is the most flavor-neutral. Sulfuric acid might be a good choice for an IPA if your sulfate isn't already sky high. You may or may not be one of those people who like the effect of high sulfate on hops, which would be convenient if you are.