Firstly, Kha-Belekh became the absolute Overlord only after the fourth eclipse.

Wasn't it after the 2nd? 564 Second Eclipse: The Rise of the Overlord of Demons.
And if it was after the 4th, then his first strike should be considered the War of the Gray Alliance, which is not among the Overlord's significant achievements.

And his first strike was flawless - the Empire of the Falcon was destroyed.

But the Empire remained intact; only the capital was destroyed. And then, an equally strong Empire of the Griffin arose.

And the Empire of the Griffin could not do anything for 400 years after that.

What was it supposed to do? Storm Shi'o? Or conquer Ashan? And where did the 400 years come from if the 4th eclipse was in 843?

omnipotence in the sense of absolute power, not all-might.

Let's say so, although in my opinion, it's the same thing. Then his power is not absolute either, because he cannot leave Shi'o; he doesn't have the strength to break through the walls of his prison.

Sar-Shazzar is not a pathetic little sorcerer, but an earthly likeness of Sar-Ilam, his student, and the prophecy said that the messiah would appear from some "virgin" (I don't remember the exact text). This virgin was Isabel, so they had to wait for her appearance.

Well, the words about a "pathetic little sorcerer" were ironic; I wrote that he is "pathetic" in comparison to the "omnipotent" Overlord.
had to wait for her appearance - doesn't this prove that the Overlord cannot (in this case, cannot do anything and simply waits)?
And, in my opinion, the earthly likeness of Sar-Ilam was Sar-Ilam himself;), and his students didn't even come close to the level of their teacher.

Kha-Belekh's eternity is calculated based on the existence of the world.

I think that the Overlord's eternity is comparable to the eternity of the elves - they also live for millennia.

Remember the single-player mission for Wier in Lords of the North, where the imp spoke about the Eight Lords:
Wier: "Marbas says that all this is true."
Imp: "Are you sure? The instructions for the ritual are written by one of the Eight Lords, the progenitors of demons?"

And do you remember the description of that mission, namely: "The Overlord of Demons is dead!", which already negates all the laws of Ashan, the further plot of Heroes, and everything we are discussing. So, one should not take those words too seriously, in particular, and the entire mission as a whole.
And the imp could have considered Urgash to be the eighth lord, i.e., 7 lords + Urgash = 8.