Per Hum's introductory dialogue, it is hinted that Incubi feed off dreams rather than physical essences as opposed to Succubi, so they are considered separate species unlike some real-world interpretations of them as male/female members of the same race.
I was able to trigger the event too, but the only insanity related event I've done before was the "Person" interaction in Hotel Vania iirc.
Gonna agree with almost all your theories cuz I'm biased (been thinking about this throughout last year) lol, this update is an outright declaration of the fact that Dilan isn't the first of his kind, that they've been known to the Council for a while now.
Maybe there is something about agglomorate essences that prevents another from integrating it readily, but the Walter cutscene seems to imply that Dilan was canonically resisting Bonnie/Katja's influence, and it was only after being pushed to his limits that Katja was finally "let in," as after this point Dilan automatically unlocks Frost magic.
It'd make sense if Svelva was actually responsible for making Bonnie into an agglomorate, too, since Katja makes statements openly resenting Svelva near the end of the Purplehead possession. If Svelva is known for being a master manipulator, the previous dream cutscenes would imply he tricked Katja into becoming his friend only to meddle with her soul.