What Is it specifically that allows you to play as an adventurer from any civ?
I ask not because I don't know about the raw file tags that control it, i do, but because of a bug(?) that appeared in the Museum III game a couple years ago; dwarves disappeared from the menu of races you can start an adventurer as, and had to be modified so they are outsider controllable, and a demon called "charcoal brutes" became playable from the same menu. This was weird at the time and still is, because there was still a healthy number of dwarves in the world, including two (now four, resurrected by dfhack) civs.
I'm starting to suspect the dwarven non-historical population got wiped out, and that's the cause of it. And i suspect that all of the dwarves alive in the save today (400 years after world gen ended) are members of player forts, caravan guards and merchants or adventurers, who are all historical figures, and that, for whatever reason, the game is not counting historical figures among the population that counts towards whether a race is playable. Humans, elves and goblins all have non-historical populations, you can meet them and make them historical figures in adventure mode. Hands of Planesgifts, a necromancer experiment species, also have a healthy non-historical population. Black bear men apparently also do, although I don't recall seeing it in the sites and pops file. And charcoal brutes... no idea how that happened. Maybe from the goblin civ that took over a civ that dug too deep.
But, I've been running an experiment to attempt to add new species to the list: I've been recruiting and breeding animal people from the wilds, and have a population of wolf people and wolverine people that both have had children born into the dwarven civ as naturalized citizens. I also have coyote and kea people that I expect to breed next turn. None of that has resulted in them being added to the list on the front page of the adventurer race selection screen (they're still playable as "intelligent wilderness people" though.)
Which is why I strongly suspect, that the races on the front page are dictated by non-historical civ populations, because all my creatures are fort members and thus historical figures.
But I have no way to confirm that that's how it actually works, hence asking here.
Also, did you intend to re-add the sites and pops file export to the legends mode menu along with the xml export?