I'm sure they could've written Shadowheart being able to be convinced into a threesome without her cucking the MC. It's the established relationship trajectory warping in the presence of Halsin which is the main culprit here.
In some ways, Larian is predictable. EA Halsin was publicly character-assassinated during the last PFH. And in game, you walk into a brothel and talk to a pair of drow prostitutes...
Larian writing + drow hookers + Gary-Stu Halsin (overriding any and all romance flags) + any companion
