First off, I would try and seek out a BJCP to sample to get more accurate feedback. Hopefully you have a local or ship to one of us. This should help distinguish polyphenol/tannin astringency vs bitterness. That said, based on your description, lingering/back of throat is harsh bitterness. Classic for Columbus if you ask me. Did you use Columbus in the successful beers? IMO NE IPA should not have any high coho hops nor boil additions. We can pick apart your process but it is actually pretty sound, including the 5.2 pH. It is now fact these NE brewers deliberately lower the pH for this style, however, the acids used vary by the brewer, but honestly I citric would work well. I would still double check your water tho, seems odd that your final pH was higher with the acid additions...