I've opted to just continue and wait for migrants. They arrived at some point in Autumn, while Mego languished in bed, being strangled relentlessly by a forgotten beast. Since then, we've lost two - Fath Olontilesh was struck down on the 25th of Sandstone, and Imush Akamgeshud bled to death on the 27th, both sacrificing themselves while combating Vemini Omethurofa.
The current status:
Ghosts prevented the construction of a carpenter's workshop by standing all over it (they're intangible! Who cares if they're in the way!?), so I built another nearby. They tried to interfere there, too, but were unsuccessful.
The current population is seven dwarves, counting Mego and ignoring the caravan and outpost liason that have shown up. We didn't have a trade depot, but deciding that sharing a fraction of the innumerable treasures we've discovered might help encourage more hands to aid our efforts, we set one up and traded away a few trinkets. To say we didn't even begin to dent what's available is a massive understatement.
So it appears that a foothold may have been established. Barring any nasty beasts from beyond dropping in to pay us a visit, we may actually be able to survive for a while, and even if they do, we have proven ourselves against two so far.
In the depths of the fort, Mego III continues to serve as a punching bag. I doubt rescue is an option, as I strongly suspect the moment the forgotten beast stops keeping her unconscious with the constant strangling, she will die.
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So... When spring hits, do I pass off the fort? Or am I finishing up Mego's turn, and then taking my own?