I guess. I just have always had an image in my head that she is just too much trouble to keep alive after fighting her, even to the point that a corrupt char will see her as a rival that needs to be destroyed rather than another potential prize. Even if you could take her in as a slave or lover, why would she give up her seat of power just to give that to you? Obviously, it couldn't be loyalty, just look at how fast Zetaz throws her under the bus when his own well being is put in danger. Beings like Lethice are out for their own good and have no interest in pleasing anyone but themselves. She would likely rather die anyways than let someone else best her in any way. Even if she initially agreed to be your subordinate/slave/etc. under the stipulation you let her live, she would eventually find some way to get the jump on you and turn the tables. To put it simply, I've always imagined Lethice to be 100% untamable and 100% unreasonable; she's hellbent on ruling, not being ruled in any way. In fact, she'd probably would let you collar her if you beat her, then sooner or later find some way to stab you in the back, or (if you'll excuse the morbid implications) take her own life since dominating her would likely do unspeakable things to her pride and sense of worth.
That said, considering what Savin said, if there is another way to remover her from power without simply destroying her already planed, then that could work depending upon what sort of option it implies. I've always though Lethice is no coward and more willing to die than lose, something that separates her from the others like Zetaz and Ceraph and could perhaps be why she is so powerful. Or, I could be way off base with how I see Lethice and perhaps she does value her life above her being at the seat of power. Still, if that's the case I think that would be more or less her convincing the PC to let her live rather than the PC simply changing their mind and letting her live in the hopes she will agree to terms. Maybe I'm just describing MY PC's, too. My evil PC Kane, for instance, would probably just kill her the moment he got a chance to and say it would just be the smartest route. Being corrupt as hell himself, he would probable screw her first though. Aura, my good girl, would kill her too, though she'd have trouble coming to grips with the idea of taking even Lethice's life. She would even bury the body afterwards out of guilt. Poor thing would definitely hate herself for a while afterwards, but she'd be thankful she had the courage to do what had to be done to save her home.
I never did play MGQ that much, though from what I remember, you do get quite a bit more personal interaction with Alice's character than you do with Lethice. So not only do you learn to sympathize with her and her side of the story, she learns to sympathize with your side too. Maybe if Lethice had spent some time with the PC, maybe the same thing could've triggered. Funny you bring up MGQ though, I even had at one point a long time ago the idea for the PC to encounter Lethices' daughter/son without knowing who they were at the beginning of the game. The idea was that he/she would act much like Alice kind of did, still being secretly in league with his/her mother, but unlike there mother seeking to understand the "humans" better. Either that or Lethice sent them to spy on you. They wouldn't be available for sex, more like someone just tagging along or observing from a safe distance perhaps. Eventually, the PC would learn of their true identity and could either banish them from camp or let it go. Depending on that, he/she would have an effect in how you deal with Lethice in the end, possibly even changing what happens to the PC afterwards. Never really got it off the ground beyond theory crafting since the daughter's/son's obvious capacity for compassion didn't make sense to me, being Lethice's little shit and all.
Anyways, sorry for the wall and all, just speaking my mind. It's been a pretty uneventful and boring week.