He Who Walks Before said he was the Gate Breaker. He's the one that leads a frontal assault.
He Who Walks Behind is an assassin.
Nemesis is like a combination of a spy and saboteur.
It's a war, and the Outsiders, even though they are totally whacked out, are conducting it like any war. Considering the whole of *Cold Days*, I think it's clear that Nemesis is one of the Outsiders, that's why GK was scanning the injured at the Gate, and he said even he couldn't be totally sure someone was clean. It's just, very apparent that it's from Outside, and it'd be a pretty big fudge at this point to make it otherwise.
The Outsiders look all jacked up because they probably have to assume some kind of corporeal form just so they can interact with the spiritual/material realms. I think that's part of what the deep hatred is all about, they see our ordered existence as an affront, and just the fact that we exists causes them to also exist apart from their idyllic chaos.
The Outsiders are completely separate from anything Inside, the books and Jim have been about as clear as these things get without just unwrapping every bit of mystery. Outsiders are from Outside everything we know, the material of the Outside kills anything it touches, no one is just going to waltz Outside.
There seems to be conflation between old gods and Outsiders, especially with the Oblivion War. I think if the Oblivion war *primarily* had to do with the Outsiders, Jim would have mentioned that, or otherwise not mentioned it at all and done the "I'm not going to tell you" song.
The Oblivion War is a nice backstory Jim added to the Archive, who was initially going to be a throwaway character. The beings that the Archive is trying to get rid of seem to be things more similar to the necro-god that Cowl et al were trying to become. That is, a load of power without purpose, just beings that got themselves all hopped-up on Power, and maybe some creatures from deep in the NN.
I'm on board with "Oblivion" just being a place inaccessible to the normal flow of things, but still not "Outside". In fact I don't think it Oblivion a "place", it's more like, they cease to exist in any manner that can act, only existing in the "mind" and "memory" of the universe itself. Jim said that Mab and the Sidhe were in danger of Oblivion until she tapped the Brother Grimm.
The Outsiders want to destroy the universe, I doubt anyone from Inside really wants that to happen. They might want to rule the universe, might want to turn it into something else, but they still want it to be here. If you want to take anything Dresden's said to heart, it's the "Mab's a monster, but she's our monster" part, the part about playing for the home team.
At the same time since it needs to be mortal magic that summons Outsiders, I could certainly see her also making every effort to keep written records about Outsiders are destroyed. I just also think that they are separate things. The books say that the Outsiders teach people about magic and stuff, I don't see how the Oblivion War could be very effective when any summoned outsider would probably be like "By the way, can I talk to you about our dark lord and savior H'Gthudfrmmr?". And then boom, the Outsider god has an anchor.