1. To be absolutely clear, Demogorgon is not the strongest 6-C in D&D. He is the strongest currently with a page under our current understanding.
2. I'll just go ahead and give the assumption that Fate characters can affect the abstract representation of chaos.
3. All of Demogorgon's actions are thought-based and/or passive, depending on which one you're referring to (for example, his Madness is all passive based on being near him).
4. The mere act of Demogorgon existing on a Plane would cause said Plane to be absorbed into the Abyss. As our current understanding has it, this includes his 6-C form, since in a neutral universe there's no counter-balance. What makes this notable is that the Abyss is a 2-A structure that actively brings back Demogorgon the second he's ****** up. It also actively attacks outsiders. And not just baseline 2-A either, we're talking fairly high into 2-A. Said Abyss has displayed the ability to lock-up a 5-D creature, given enough time (granted, it really only contained the 4-D part of it, but it did indeed just grow around the thing).
5. What does homegirl actually have to put him down? It isn't as if, say, she has any of the survivability enjoyed by Demogorgon. The guy has Type 8 based on the Abyss, can just make more versions of himself, Mid-Godly, so on and so forth. Meanwhile she has no regen and two types of Immortality, one of which is Type 5. Which would be good, assuming EE wasn't as easily accessible as it is here, and the fact that Type 2 Concept Manip means introducing the concept of death isn't actually hard. One could argue Higher-Dimensional Existence of her mind introduces an interesting situation where her mind is safe- if that were true! It's unknown how but characters of the Planes separated and contained
Pandorym's mind and body, who is indeed a Low 1-C creature.
6.
I don't really follow controversies much but is the Fate 2-A stuff still as controversial as it was or...?