The problem is that, at the end of the Rebellion that toppled Kadaena, the Duskwalkers did not know if any other follower of Kadaena had survived (otherwise, they would have set to destroy them at once instead of going to sleep). Furthermore, the idea was not to keep all the surviving Althans imprisoned, but only to ensure that any surviving follower of Kadaena could not escape *before it was destroyed*. Other Althans (those who did not share Kadaena's twisted view) were not considered.
And there's your flaw in the plan : in order to fulfill that goal, you have to
- be aware that an Althan K'Taa'Viir has survived
- be able to know whether they are one of Kadaena's minions (in spirit if not explicitly)
- have enough strength to destroy them.
And the Daenku Ahrenreth was supposed to do that *and* close the remaining Shadowgates.
And we all know how well it went.
So by the time Daenku is alone and decides to sleep again, he also knows the ultimate goal of the Ahrenreth (destroying remaining K'Taa'Viiri who were part of 'the other faction') cannot be achieved anymore unless he can somehow rebuild the Ahrenreth - the only thing he can still try to do is ensure they never escape. Of course, he can still try to kill one, but he is the last of his kind: if he dies, there will be no guardian left.
So he sleeps through the later part of the Interregnum (basically from the last 80000 years). It is notable that the machines do not awaken him during that period - presumably because they detect no major threat. Either the machines cannot monitor the other hemisphere, or the K'Taa'Viiri on the other side have not awakened during this time (or are very quiet). And after his awakening, beyond his drive to help new races prosper, he is soon busy with the appearance of first the Dark Gods, then the Unlife. He is kept busy until the Wars, where he has to kill another K'Taa'Viir (the Master of Malice), which drives him insane. And he discovers the state of the Eastern Hemisphere *during his period of madness*, and realises that there's not much he can do all by himself. The theft of the Northern Eye is just the cherry on the doodoo sundae, but it has a cascading effect for him : if the Eyes fail, the rebellion has failed, and another Empress (or Emperor, let's not be sexist here) will rise and conquer the stars sooner or later.
So yes, he does not have a plan beyond "guard the Eyes", because he cannot find the power to do anything more - the Master of Malice was only one, and had less power than his eastern brethen, and the fight almost cost him his sanity - and he knows the threat he would be if he was truly insane. He dares not die, and he dares not become mad again - so he tries to gather strength enough to undo what has been done by Ondoval, and then... and then the Watch will resume.
In a way, I think the appearance of Jenkyna is almost a blessing for him - she is not a Duskwalker, of course, but she is not a Kadaena groupie either. She can provide strength if pointed in the right direction. She cannot be left near the Shadowstone, of course, because the temptation would probably be too great, but she's as much a potential asset as she is a risk (and better her than Ondoval, Shreck, or most of the Inner Circle at the head of the JA, anyway).
So, no, he is not insane. Probably not very hopeful, but dutybound, if anything. For him, there is no victory, only eternal watch hoping for a miracle he cannot fathom.
What is also interesting is that, after Kadaena died, the Eyes were set, and the Ahrenreth established, by Utha himself. Who then suspiciously disappears from the timeline. He was *not* part of the Ahrenreth, and he was definitely alive at that point. But then - nothing. He builds the prison and sets up the Wardens, then blinks out of history. He was Kadaena's equal, at least - probably, at that point, the most powerful being alive in the universe, with whatever Althan technology was left (and there was probably quite a bit). A god-like immortal.
And if *that* is not suspicious, I don't know what is.
