Fulminato , You're a peach.
Now, please understand that, even though I called you that, I don't intend to bury you underground in hopes of growing a peach tree. Nor do I intend to consume your ripe, sweet flesh to nourish and sustain my own body. There are these things... called metaphors. Or similes, or whatever. Point is, I don't think the Anak wants to put Xestris in a pirate chest and bury her under a big "X"!
Sorry, had to get that out of my system. Serious talk, now.
Anyway, I get it. You don't think the Anak valued Xestris, at least not as more than a means to an end. And it IS possible that he put on a false face, working for centuries against his own ends, all for one critical moment. Having met him (in game), I...can believe this of him. But just pulling a rubber mask off of him and shouting "It was old man Xerces, all along!" seems a bit...Scooby-Doo, don't you think?
No, let's give the Anak the benefit of the doubt and assume that a major NPC was written at least as well as, say, Tyna.
Let's put forth an idea that likely won't ruffle any feathers: The Anak is very old.
Now let us add a commonly held truism: Time tends to change people.
I posit that the Anak's original intent was relatively pure, even idealistic. He wanted to make things more balanced. He wanted to do it on a systemic level, not putting faith in leadership, not even his own. He wanted a world that didn't need to start a war every time a new king was crowned. He wanted a world at peace.
Now, how he got his shard, I don't know. How he ended up in charge of Philon, I don't know. Doesn't really matter for the part of the story I'm guessing at. Let's move on.
Anyway, administrating a nation is no small feat, especially alone. So, at some point, he takes a "citizen of Philon" (and more should probably be said on that subject, I think.) and adds some demon...stuff. And then sugar, spice, and chemical-X! Boom! Xestris. Now he has someone to delegate to. And to fuck. He probably talked to her once or twice, too. Who can say?
So, now Xerces can finally ignore the real world and start doing work in the super-real world. He can finally tweak everything to a perfect balance and make a better world without one sword swung or one person subjugated. Except "real" life keeps demanding his attention. And what's worse is (I'm betting) that his carefully balanced manipulations keep falling apart when he doesn't actively maintain them. And he doesn't want to keep holding down a button for the rest of eternity, he wants to fundamentally change the system. So he keeps fiddling and tweaking and worrying away at the threads of reality, never quite understanding why they keep flowing away from his intent (All metaphor, Fulminato, I'm not trying to say that Xerces is an interdimensional tailor, or whatever.). And still being occasionally pulled away to deal with, y'know, nations n' such. The trifling little details that get in the way of reality. Ripples in an otherwise peaceful gray...
I think that the Anak eventually got to the point where he was no longer satisfied with slightly altering the system. Or rather, more specifically, I think that Xerces eventually got fed up with trying to change a system that kept trying to change back. I think that Xerces, over the years, started to realize that he had an enemy. And that his enemy was, well, change. Or maybe human nature, or fate, or "the Divine Plan". Whatever the storyline needs it to be. I picked "change" 'cause I think it's the neatest idea. Makes a fitting foil to that whole "peaceful gray" motif.
Anyhoo, Xerces eventually comes to the conclusion that his initial plan won't work. You can't just flip a few switches in the Tower and have it stick. There's pushback. His changes need to be more fundamental. He needs to go...deeper. He composes a more drastic plan. One that maybe requires the sacrifice of his own life. But this would leave Philon leaderless and Xestris alone. So he would need to ensure that both were set up to weather his loss as best as possible.
And I disagree with your assertion that Philon, even a heavily fortified Philon, is adequate protection from all the forces the Anak can see arrayed on the field. Maybe people would leave Philon alone if it was just arable land and industrious citizens. If heavily defended enough, that might be too small a prize to go to war over. But if Xestris was still there? A demonic hybrid with (potentially) a whole bunch of interesting black boxes in her construction? Someone with intimate knowledge of his (old) plans? That is too rich a treasure to leave behind.
But that left him with what to do with Xestris. And if all your theories about not valuing her life are correct, then that still leaves the question of why he'd leave such a valuable playing piece on the board to be collected.
Not when he could just kill her.
Seriously. He's not powerless. Or stupid. If Xestris had lived out the end of her usefulness, why would he send her on a fool's errand that only might kill her? Why not just say "Thanks! Oh, also hold this!" and just stab her to death while watching the life slowly drain out of her eyes? He made her. Who's to say he couldn't just unravel her? Hell, you've seen how loyal she is. She literally defends his decision to throw her away. Why didn't he just say "Excellent work, Xestris! Now, kill yourself."? There's at least a 50/50 shot she was loyal enough to kill herself on the spot (Well, barring Nalili showing up).
But, no. He sends her on an obvious suicide mission against the Lust Lord. Cutting ties with her as callously and as suddenly as possible. Literally saying "You don't matter.". That's more words than I personally would waste on someone who doesn't actually matter to me. If she actually was just a utility to me, I'd do what most sociopaths or narcissists would do in this scenario and just pat her on the cheek, tell her to try her best for her dear Anak, and remind myself to check her corpse in a week or so to see if she managed to get anything useful before she died.
Because that's the thing. They worked together for more than a century. That level of proximity and time is...it has a profound effect, psychologically. Hell, even neurologically. You can't work that closely with someone for that long without a measurable amount of your grey matter being devoted to maintaining a predictive model of that person. Just being able to chuck all that away without a thought requires more than just "being an edgy-cool guy". You need a fundamentally cracked brain. Acting like you don't give a damn just requires acting ability. Hell, I can do that. All things being equal, it is far more likely that he cares about her than that he doesn't.
And finally, I've read your post, like, 3 times now, and I still don't understand what you're talking about in regards to Esthera and the Doom King. The Anak doesn't need any special knowledge to assume that Xestris will end up in Simon's employ if given even half a chance. He just needs pattern recognition. He doesn't even need to know anything about Simon and the Doom King. He just needs to look at Iris. And Riala. And Orilise and Lynine and...I guess, technically, princess Reletima, and...
Look, Simon picks up politically influential subordinates like magnets pick up iron shavings. Expecting Xestris to end up in his pocket (again, metaphorically), is not a stretch of the imagination.
(A side note: I typed "the Anak" out so, so many times. And the "L" key is right next to the "K" and spell-check doesn't even pick that one up. So I apologize if I missed any instances of me talking about "the Anal". Just assume Yarra stole my keyboard for a moment.)
EDIT: Also, I read some of your other posts after typing my thoughts out, and we actually align pretty closely with our analyses of what is known so far. But I still think the Anak dots the "i" in Xestris' name with a little heart whenever he writes about her in his diary. Or, as he calls it, his "Incredibly Devious Plan to Rid the World of the Tyranny of Free Will (No Peeking!)".