Quite a heated debate you got here, But i think you are making a better deal of this than it actually is because of 1 ability. I understand you want to make everything beatable but this actually isnt as big of a deal as you think it is.
"Herald of the End (Ex)
The Nowhere King was brought into existence by the creation of the universe, but was the living embodiment of the void, the perfect nothing that existed before it was born. The emptiness that it originally reflected had itself been erased at the start of time, and it longed to return to the ancient calm of nonexistence. The Nowhere King gains Uncanny Annihilating Mastery (x2 HD) as a bonus ability. Creature killed by the Nowhere King are destroyed in body (the remains are annihilated), mind and soul and cannot return to life or be resurrected by any means by a creature with lesser grade of a First One. This ignore rejuvenation and all abilities that automatically return to life or restore a creature (like the gaseous form of a vampire, the phylactery of a lich or the necessity to be killed in the Home plane for Immortals). The Nowhere King can select Transcendental abilities as normal abilities. This ability cannot be abrogated."
K so first this ability basically just gives it the uncanny annihilating's effect x2HD which you might think is a Huge deal but NOWHERE in this does it say the Annihilating effect is changed. So, its still a deals D6 dmg, which considering an average first one party has hp in the D100s, this isnt a big deal. And the annihilating's permanent dmg can be healed by Wish which as first one they can do for FREE EVERY ROUND.
And if you read the text, the kill any thing permanently thing CAN be overcome by first ones, which the Party is. SO if one of them dies they can Resurrect him AS NORMAL since they are First Ones. Sure it may take a Epic Life seed but thats nothing at this level.
Lastly the select transcendental ability is useless because those abilities can be abrogated as normal and it would be a waste of an action to switch mid combat.
So no, its not that broken.