I'm likely going to plan a week or so vacation there at some point in the next few months with my gf, when I do that is it fine if I ask you a few questions about basic stuff...? (mostly advice about which places to make sure we don't miss)

Absolutely.

Depends what you call the core. The dense pre-WWII area of the city is sizable enough, but it's pretty ordinary by Northeastern standards (typical example below) and nothing special (unlike the actual old town which has somewhat urban stuff from the 1600s and 1700s).

Having plenty of neighborhoods of this is decent from an urban point of view but it's not going to make you a tourist magnet:
https://www.google.com/maps/@46.8099...7i13312!8i6656

I don't think many visitors fly but yeah, there are near-invasion-caliber volumes of NY/PA/NJ/New England license plates, almost year-round (but especially in the summer). And you hear English all the time in the touristy areas.

In this respect, it's actually not comparable at all to the U.S. at all... In the U.S., the pool of people who could choose to move to any desirable city is basically the whole country; in Quebec, the entire pool is pretty much limited to Quebec (i.e. French Canada) and we already have Montreal that has good urban qualities and lots of historic architecture of its own (and more jobs and more HQs, etc.)

Imagine that you have only 8 million Americans in total, of which 4 million live in New Orleans metro, 1 million live in Charleston metro, and the rest live in smaller cities and in rural areas, while everyone has the same climate and everything's within a day's drive of each other.

In this context, do you think Charleston would have anywhere near the same appeal as it does now?

Thanks! :)