but I also feel it would still have been the right decision to go to an ER, because it isn't like we have metoclopramide lying around the house.

Very definitely. And unless someone's already got a pattern of migraines with that exact set of symptoms, you don't know that's what it is, because it can and does look exactly like other neurological events, and there's no way of working out which without the scans.

It would just have been nice to be able to say to the triage nurse straight off, "This might be a migraine,"

YUP.