The details of Stelmane's 'stroke' :
This info is wild! No wonder he chose her; she is like him in a certain way. But I'm curious, "Since she awakened, her mind has been a constant battleground between her own psyche and the illithid—except […]" did he never succeed in fully enthraling her because of her exceptionally strong mind, because he didn't have enough energy to conduct the ritual to its end, or/and because he didn't really wish to erase her personality?
Its house tour and the conversation about Stelmane have elements of this but they are fake, it never cared for Stelmane and the things it shows us during the house tour is basically trash. Even if the objects had sentimental value during its human life, they are just random objects now.
The portrait of Stelmane is a generic picture, there's no bed, no chest with very confidential maps or documents, no Qualith tablets or illithid magic stuff. I am unsure whether the hideout's design has been rushed or if the actual private room is intentionally absent.
We cannot directly confront the Emperor about his business partner, but he has already expressed his sentiments through a previous dream. I believe he truly 'cared' for Stelmane as she was his main tool for a decade. All the things he says about her should represent a genuine side of the coin, as he simply avoids acknowledging the dark aspects of their relationship (which is monstrous from a human perspective. Especially if the enthrallment procedure provokes an unavoidable stroke followed by permanent damage, worsening with: "Nor has she found a way to break its grip on her sufficiently to signal for aid.")
The Emperor's attempt at seduction mimics the development of the PC's romance with their chosen companion
This scene doesn't work for the Tav who never treated him as a human. Dev tried to adapt some dialogue lines, but for me it's clearly designed for players who resist the astral tadpole rather than to create an emotional bond and gain their loyalty or to push the half-illithid Tav to desire to achieve transformation.
Speaking of bond, why doesn't the emperor keep his human/illithid 'lover' at the end of the adventure? Shouldn't these Tav be forced to follow him anyway?