Update:
I am a monster. Or a dwarven scientist. Probably both?
I had stopped getting migrants and realized that it was because the feral remnants of my civilization down in my basement counted against my hard population cap, and developed an appropriate solution:
Fortunately, the cave level isn't too far down (about 78 levels) and doesn't open up until near the bottom of the world, and then immediately flows off the map. Piercing an aquifer thus isn't too much of a drain (ha! I jest) on my performance.
This has met with considerable success:
I've been getting regular migrants at the scheduled times again now, which is a definite improvement. Also all the mud I've been creating is going to make my wood industry
amazing.
Interestingly, the dwarven caravans seem to be coming every year again, and claim to be sent by The Fortunate Banners. This doesn't seem likely, since the outpost liason is currently treading water inside an air pocket in a sump, I AM the mountainhomes, other Dwarven cities were wiped out 2400 years ago, and we're so far out in the backwaters of civilization that even the kobolds and goblins haven't visited us in 2 years.
This has, however, produced a complete halt in the productivity of my craftsdwarf workshops, as all four of them are now cranking out memorial slabs to the CHUDs who keep drowning down below.
Future archaeologists conclude that this must have been a seagoaing dwarven civilization.
So far I have not managed to claim the lives of the wily outpost liason or any of the nobility (as far as I can tell), however some further monstrous science on the surface has demonstrated that I can send my military after individual dwarves and the others don't care at all. No wonder the goblins of The Virginal Hatred had no trouble wiping them out repeatedly.
I'll be attempting two pieces of science here: I'm going to wipe out all of the hostiles except a single baroness (if possible), then retire the fort once migrants bring the population level (and the fort's food production) up to a decent level.
I'll then take a look at the legends mode export and see if everybody on the site has the same entity population id.
Then unretire and see if the baroness is now part of my glorious civilization or if she's still living down in the sewers eating crocodiles and bringing down the overall classiness of the joint.
Edit:
Lab report: CHUDs remain CHUD-dy after unretire.