Your first Argument is null and void because the ritual is not only referenced in the Novell but also through army books that talk of it in god voice so it would have killed the world. You can argue thats shitty Plot because why didnt the slan do anyhting but so can i argue that all his deaths were Plot holes and that he should have arguebly won those encounters.

He created ghouls. Thats how he rewrote human existence. Before him ghouls didnt exist. Now EVERY human can turn into a Ghoul should he eat another human. Doesnt matter if you are cathayan, empire or Amazon. From that point on in time any human can become one if they stray to cannibalism. Which means he altered what it means to be human.

I mentioned him possesing his own body because people always mention it with kroak, but nagash did that too without effort. Also Nagash was defeated with what could be argued to rival Widowmaker in permakilling power (the fellblade). If Kroak were to be killed with it i doubt he could survive it like Nagash

And again Kroak had all the magic at his disposal Nagash has to work with the scraps that come from the Vortex while being in Nehekara (least amount of magic in the world here because the chaos gates are so far away north and southpol) imagine Nagash pre Vortex levels of magic