I did find (here) Mystic white being offered as an interior option on some squares. And I saw the SEM Mystic marine paint too. I wouldn't expect it to match or not match, I'm pretty sure the common name is a coincidence and doesn't mean it's a GM match. Do a search here for Parchment interiors, you may have to search the whole site and not just squares. It's an off-white color and SEM does have it. That's probably a good option. Are you getting a new headliner? SEM Sailcloth white looks interesting but it's pretty white. You wouldn't want the side trim to be a brighter white than the headliner, so I'd probably buy the headliner first.
I'm looking at SEM colors here:
LINK
I've searched here and did not see a thread about the correct SEM interior paint for square white interior trim. I'm sure the info is here somewhere. But for now I'd buy Parchment if you're going for "correct as possible" and maybe Sailcloth white if a brighter white sounds appealing.