Under the cut, because this got long.
Summary
For F’s accent…It’s possible that they do have a southern accent. “Doolally” isn’t American slang at all, but they still say it. Also, I’m technically a southerner, and stuff like “bless your heart” and “darlin’” is more deep south. The South is a huge part of the country.
We don’t really know how long F has been in our world, other than it’s between 10-80 years (though probably less than 80). If they arrived in America from the Echo World, it’s probable that they lived there for some time before being selected as an agent, since only the strongest supernatural are chosen. The accent is likely part of the Agency’s efforts to help them assimilate into our world, and the name possibly as well. If the name was chosen for that purpose, then it doesn’t reflect their heritage at all. And just because it’s French doesn’t mean anything in America, really. I live in WV and I’ve had classmates all my life with French-sounding names, and I promise WV isn’t full of French culture or that of their descendants.
As for M, they might be Turkish from before the fall of the Ottoman Empire. Since we know F is ~30 years older than Rebecca, who’s probably in her 40s/50s, that puts F’s birth in the Echo World around the 1930s-40s. We know that M is older than F, but I’m hesitant on making them more than a few decades older, since making centuries also bumps up N and A’s ages. The fall of the Ottoman Empire happened in the 1920s, so M could be from before that. The Ottoman Empire didn’t use surnames, it’s in the Mediterranean, and they had rock crystal.
Rock crystal, really just quartz, held some sort of significance to the Islamic religion (I’m not Islamic, so I’m a little fuzzy on this) because of its translucence. Maybe that’s what M’s crystal is for. Does anyone remember how it was described?
I was going to say something about N’s lack of accent, but @Celtic_Rune beat me to it.
And I don’t speak French, but mort means death. So either A had a real ironic family name or they have a weirdo sense of humor. Since the names tie in with backstory, probably the first.