The quest to stabilise the updated 40.15 fort has made some progress. I had to crash and restart several times, because the goddamn "tantrums cause loyalty loss in witnesses" monsterbug still occurs in 40.24. Twice, a very unhappy dwarf threw a tantrum in the meeting hall and everybody who saw it started murdering everybody else. I finally totally and utterly locked up the tantrumbastards and dumped some food and drink in their confines. Had to do it all over in one case, because the living timebomb decided to climb through the dumping hole before i could shore it up.
There are still two free-roaming haggard dwarfs (the haunted one finally snapped and starved herself to death in melancholy), but they consistently don't tantrum and since the save has been imported to the new version, sadness from distant snatched relatives no longer accumulates, so the situation is actually improvable. I have over a dozen dead from .40.15, all but one dead from dwarf-on-dwarf combat, almost none of them the actual tantrum-throwers.
I had earlier tried shorting out tantrum-triggered loyalty cascades by retiring/reactivating the fort, but that led to the volcano overflowing and eventually a game crash. So, full-out save scumming it is (i'm not too torn about it, because the bug is just unspeakably dumb).