I agree it IS very much like a two-parter (more than any other TNG movie), akin to "Gambit" or "Time's Arrow" or "Descent". It wouldn't seem out of place if INS had aired as a two-parter during the 6th or 7th season.
And yet, I'd still feel very "eh" about it. It's a step down from "Time's Arrow", for example, and even that one isn't too hot.
INS would be routine if lackluster as a two-parter, with some significant problems. As a movie, INS sucks.