"Vivec can activate his CHIM while being in a erased state, in fact to achieve CHIM in the first place you need to be erased"
Okay, while you are correct in that the entire thing with CHIM is that you get erased but your very will allows you to overcome the erasure, and the erasure in question that you must overcome involves stupidly high degrees of outerversal complexity (
https://imgur.com/a/y2PrMmb), it would be a pretty big no limits fallacy to assume this means he can enter CHIM through any form of erasure across all of fiction. Why? Because Zero-Summing is a very specific thing, and the achieval of CHIM is less to do with being erased and more to do with the nature of Zero-Summing itself while still being able to see the Tower sideways. To further my point, even those using CHIM seem to be subject to World Denials to some extent, even though they can sort of reform with their Regenerationn and immortality.
"My knowledge on TES is limited, what exactly is nymic? (Sorry if you get asked this alot)"
A nymic is basically the essence that makes up your being within the Godhead's Dream, and is a huge key factor when it comes to the nature of Mantling. Being able to influence someone's Nymic allows you to rewrite their very existence at the most fundamental and primal level.
"Caine erases Vivec on a level beyond all things and narratives.
Vivec goes to regenerate and goes to materialise
The exact moment Vivec regens on any level, he's erased again.
Repeat ad infinium"
Right, one issue
"You would have to erase Amaranth to erase Vivecs narative."
This is true, and very much an issue for Caine I'd think. Everything is ultimately still a subgradient of Amaranth, and so their narratives are one with Amaranth.