I have a world with exactly one dwarf in one of the civilisations (I'm doing a Youtube tutorial/let's play on it). Basically, he was 2 when the civ was destroyed, but managed to survive. At the age of 12 he became military commander. 110 years later I started the game. Some interesting points:
On the legends history map, there were still areas claimed by the civ, despite that fact that both of their sites were ruined (one was reclaimed by the other dwarven civ). I believe that when a civ is truly dead (i.e., when all the members are dead), there will be no areas on that map claimed by the civ. Looking in the sites_and_pop text file exported by legends mode, I could see that there was still one dwarf living in the ruined, unclaimed fortress.
However, in another world, I have had members of an almost dead civ living in a goblin town. They subsequently became queen of the almost-dead civ, while simultaneously being a member of the goblin civ. After looking for other examples of this, I believe what happens is that when a dwarf becomes a disciple of a master in another civ, they gain membership into that civ, without relinquishing their own civ. So you can have a member of an almost dead civ hiding away in a dance troupe, or whatever, until they suddenly become queen. Potentially it can go the other way, where a goblin would study under a dwarvish master and when the dwarf civ got destroyed, the goblin could become king of the civ, while still being a member of the goblin civ. I've actually heard reports of goblins becoming kings/queens of almost dead civs, and I suspect that's how it happens.
Back to my almost dead civ with the millitary commander living in the ruined city. In my let's play, I started an adventurer to go and find him to interview him (with the idea of trying to write a book about him -- not sure if it is possible). I just noticed when I looked at the updated legends save that after I started my adventurer, he suddenly became baron and then king of the civ. So it seems that isolated dwarfs will not become king in world generation, but will become king once world activation starts.
I am considering starting a fortress in this almost dead civ after I meet with the now king, but haven't decided exactly what I want to do. It would be nice to allow the king to reclaim his ruined city, but since there are no dwarfs, I think it is impossible. The only way to do it would be to retire my fortress with the hopes that they would launch an expidition to the ruined fortress. Or I could reclaim it myself....
And just to verify, I can not make an adventurer in this near-dead civ, but I *can* (last time I looked) start in fortress mode with this civ. Not sure what would allow adventurers of this civ in the future...