Same thing, really. If one falls, they all fall, so they're destroyed rather than allowed to reach that point.
The relevance of the Dresdenverse would have to be that it's one of the last.
My thinking is similar. There is a story line in the Lucifer comics (not the travesty of a TV show - I understand it might be enjoyable in it's own right but after reading the comics...yikes, complete miss), that Fenrir attempts to destroy all of reality by attacking Yggdrasil (one of the beginning places). If it falls, all of reality would. I can see something similar for the Dresden Files. It's not enough that Dresden's universe might fall, or might even be one of the last remaining, but the Outsiders are intending to destroy something fundamental to all Creation. A pillar of reality. If they fail to stop them, the Outsiders win everywhere.
I've always figured Mirror Mirror will end with Uriel showing up and escorting Harry back to his timeline. Depending on how things go, it might be even more shocking if Harry sees the other timeline, and at the end gets pulled out by Uriel only to watch as a spectator as the timeline is obliterated. And then Uriel turns to Harry and says "there aren't many left".
Talk about pressure.
Are you saying you think they blinked at Harry and then spoke through Vadderung when he couldn't?
I lean more towards it being Vadderung regaining consciousness a few minutes before, and triggering it when he thought it'd help.
I see H&M being non-combatants that help Vadderung stay weak enough to remain involved.
Wow, that would be a great scene! What a set up. Nothing to remind Dresden he's involved in a high-stakes game. Not to mention, it fits the pattern. Summer Knight, Death Masks, Dead Beat, Small Favor, Changes, Cold Days, and finally Battle Ground all set the stakes higher and higher. All were potential apocalypse scenarios had Harry not stopped the villains. They increase in magnitude and violence each time, and Harry learns things are even more on the line than he thought. So I can see Uriel opening up the problem to Dresden and saying "You thought you had it bad before...you had no idea" or something along those lines (in a more Uriel style).
Ferro
I don't think he would in the current climate, but why take the risk?
I am sure Drakul wouldn't be too concerned either, but he seems to be a long-term kind of player. All the really dangerous guys don't miss. So if Ferro were to fight Vadderung, it would only be a fight he was sure he could win - unless he was desperate. Vadderung is exceedingly dangerous. He almost plays chicken with Ferro in Peace Talks. "Take a swing if you want, but make it count, because otherwise I'll take my swing and I'll put you down". It's that sort of thing. Immortals who have lived a really long time would have to be somewhat cautious by nature. Against puny mortals I doubt they get too worried, at least in smaller fights. But against other similarly powerful beings I am sure they're a bit more wary. Just on the off chance that the other guy might just have done his homework and is ready to kill him. Immortality is a hell of a thing to lose.
Also, I don't think the body guards were involved in the fight - at least, not at the Ethniu bit. They're sort of like bodyguards but they're also his agents. I wouldn't be surprised if they were doing sabotage or recon on the enemy while Vadderung was kickboxing with Ethniu. Makes for a hell of a distraction.