Also Adam and Eve would have nothing to do with ancient history with regards to billions of years ago.  He has Archangels, TWG, souls...  Adam and Eve are part of that belief system, and their Choice led to the fall of humans.  The Angel's theme throughout the series has been about Choice, and Harry is the central person of the stories which means his Choices are probably what will decide the fate of things.  He's also a Starborn which seems to be of importance.

As far as we know, his magic can affect Outsiders, for an extent, so a Starborn would be integral to the war.
However, TWG and Archangels in DV seem to have two different roles. According to Dresden(a bit), Michael, Uriel and their bunch, He is capital G God. His Archangels can level galaxies. (Even though they can be scared to death if they're pet-named)
According to Basserina, and some other nasties/not so goodies, he is but one of many small g gods. He just gained the most influence, following, and thus power 2000 years ago, and all but drove the other gods to dust. Yet they still exist, and trying to regain some following.

There is also the fact that Hades doesn't have many believers anymore, and thus not many souls go to his dominion after they die, but those who do, he has the power to judge, and keep, for eternity. TWG has no power over that. Which makes me lean towards the "The most powerful of small-g's" theory. The attributed super powers are just PR.
Edit:
I'm getting the hang of the initialisms thing.

Tough to tell...  She hasn't had much face time in the books, and she is somewhat mysterious.  Harry thought she was dead for a long time...  She was completely off the grid.  The Council doesn't know about her, Harry doesn't really know much about her, and neither do we. 

If she were Nemfected/Under Cowl or Justin's control, why would she save Harry and then give him a chance to disrupt Aurora's greenpeaceageddon? Doesn't make sense to me.
She also openly admitted to Harry at the beginning of Summer Knight that she was Justin's thrall. That would be counterproductive as Harry had no reason to suspect it to begin with. I agree with raidem, she's not under any known influence now.

I think this is possible mainly because it is something of fantasy standard.

I remember in WOT, Rand al'Thor used a flawed S'angreal to overcome Ishamael's will, use his magic for his own, then ultimately giving him the boot and taking his body.

In DF, Harry used Little Chicago to leave his body behind and walk around as a spirit in Chicago, haunt somebody (heh) and obtain information. Cowl attacked him with a psychic punch, same way as you'd attack a ghost. I'm willing to bet that had it been Capriocorpus or Molly there instead of Harry, they would have been able to possess Grey Cloak.
So, in my opinion, it's already been done, the basics anyway, and all that's left is work on the prototype, make it more mobile and less location oriented.

I never could understand that part about Harry Potter universe. Why do they have to use a wand? They do it unconciously, or not so unconciously untill they begin their schooling. If it is done completely unconciously, then it makes sense that Hogwarts teaches control over their magic, so that it doesn't accidentally happen. Once they learn enough control, they should be able to let it loose if they need it to.
On topic:

• If potterverse operated in the Dresdenverse, Hogwarts will have to start Using Straight line depcreciation.
• I'd imagine Hogwarts witches and wizards will have to learn how to aim. Maybe add some crosshairs (Why didn't they? Never could figure it out).
• The Red Court would have long been wiped out, as the main problem in the war was transportation/communication.

Terribly wrong, you are what you eat, doing a Hallow and remaining entirely yourself is nigh impossible i'd bet. Your eating a bunch of spirits.

Heh. Heh. Regretting all the Dirt eating dares from my childhood now. It explains a lot though.

In the Dresdenverse there is right, and wrong.  Good, and evil.  Angels who violate laws fall. White Court/Red Court vamps must kill to transform, and become 1 with their demon.
 The vampires thinking they are innocent doesn't matter.  In that universe they are evil.  The only exception are those very few who are like Thomas who fight their evil demon. Very few have shown any care about killing though.

True. I'm not contesting the fact that they should be exterminated.

Nothing influencing your choices but the normal side-effects of performing a massive sacrificial necromantic ritual.

The resolve though. You have to believe in what you magically do, to pull it off, you have to absolutely believe that killing a few people to achieve your goals (whatever they may be) is right. It's unlikely that you would change.

but at least it's you paying the price rather than a bunch of dead innocents you had to drain.

Isn't DarkHallow supposed to be about sucking in spirits to increase your power? (Drunken Master. Heh) The vortex sucking life is more of a byproduct.
The Fallen will have more power over you, influence your thoughts, and eventually turn you into a monster that would just love to cause a Mini-Apocalypse. With DarkHallow you're still your own man, with nothing influencing your choices, so its more of a "Right-now-evil-act" vs a "Later on- a -chaos-loving-maniac" thing.

Come on, that's not the same as a DarkHallow.  There is a difference in killing people for power, and casualties if war.  His targets were the evil Red Court vampires, the innocents that died as a result weren't targets, but other victims of the Red's who'd been infected without permission.

I agree with your first point.

I think the innocence being talked about were the people infected, but not turned who died.  Only the younger once's survived I believe.

No, he's right, I meant the vampires. And from their perspective, they're innocent. They have to feed in order to survive, they see humans as herd animals, so they have no qualms about slaughtering them to feed themselves.
Yes, they still have to be killed on basic principle, it's an "us and them" kind of situation. Wiping out an entire species was a concious choice for Harry, fueled by need and much anger. He was quite unhinged in Changes, and DarkHallow was a step away. That's still a moot point though, I can't see how he could manage DarkHallow in such little time, he didn't prepare the way as the Kemmlerites did in Dead Beat, no collective fear, necromantic energy to protect him, and with a broken back. At that point the only viable options were the Coin or Mab.

Fun, isn't it?