So the new lower fascia is installed and it did indeed close the dreaded gap somewhat, just not entirely:
I'm heading over to the shop later this afternoon to inspect the vehicle. In the mean-time, my insurance "physical damage expert" person is going to make some calls to figure out what on earth causes this and if it's common in the nismo model.
At this point if it's not really ugly, I'll probably settle for some "hush go outside and play" money from the insurance company and call it a day.