He honestly cared for her and has regrets about how things went. He kept personal items of hers in his old den, even a portrait of her.
What if something went horribly wrong, and she suffered a horrible attack from a different illithid. However, due to her position, and his need of her as a public face, he needed her to still remain functional and operational while she struggled with the mental assault. Yes, he would be puppet and using her in her hour of need, it also served to protect them both.
Such a turn of events is a curious idea that could be nurtured into a deliciously painul story... Buuut in reality it was probably somehow tied to the Order having dark cultists in its ranks. The Emperor seems to have this habit of getting betrayed by his closest allies during the hardest of times (Ansur and quite possibly Tav), so perhaps Stelmane was one of them. Or maybe she used to be this good, kind person once... but then one day she got brainwashed into becoming a follower, and in an attempt to bring her back to her former self the Emperor gave her a stroke instead. Maybe she was on the dark side all along and managed to trick him, so eventually a time came when he found out about her being part of the cult and had to mind control her in order to survive. Alternatively, he could use her face and influence to keep the cult from actively spreading and harming his city (making all those morally grey decisions along the way. He does try to lean to the 'good' side, after all). Or perhaps she was always pure, and he's just an evil bastard with a twisted understanding of their relationship, though that doesn't seem to be true to his character.
This. It would be really nice to see this better explored and explained. In my perception of it there is a lot of unknowns. A lot of choices he makes in the campaign feel like they are done out of desperation, and that he is willing to do what it takes to save the city he built. He seems like a long game calculator, that he understand survivor comes first so that he can fight another day. The whole aspect of lose a battle, but aim to win the war.
