I had a theory that the problem might be in the number of sims who have to be update every time a baby is born. With that thought in mind, I looked around for an available bachelor in Xanadu and a mother of one of Gregsim's children who happened to have a vacancy.
The result:
Justine de Sade with Jezebel, her baby by Bryce McNab
Justine already had two children by Greg, teen Juliette and Marguerite, who grew up to be a toddler while Justine was in labor with Jezebel.
I was wondering whether the whole neighborhood was jinxed but clearly, it's not the neighborhood.
That leaves the question of whether it's possible to create a new sibling for Gregsim's multitudes. Justine, being Justine, was eager to volunteer to find out. So:
Justine is pregnant for the fourth time.
If she can deliver this baby, then that will narrow the problem down to just sims who were pregnant at the time I installed Nightlife and Open For Business. That could have dire implications for other neighborhoods.
If not, then I'll know the problem is specifically related to Gregsim in this neighborhood, though we can only speculate what went awry. I am suspicious that it's because so many sims in so many households have to be updated every time his offspring get another sibling. He came here with his whole family so his parents and grandparents exist in Xanadu, too.