Again, the Darkhallow summons ghosts and that's what the user consumes. The Sleepers die in the aftermath where they are sucked into the vacuum caused by it.
You know, I am not so sure that's the case. If you read what the Darkhallow actually does back in
Dead Beat, it's a spell that consumes spirits. I am not sure it distinguishes the spirits between those that are shades of the mortal departed or anything else. Bob was potentially going to be consumed if Cowl had succeeded, Dresden notes. Bob is no ghost or spectre, he is an air-spirit, a demon by certain definitions.
That said, I suspect it is impossible to consume an immortal without them first being made mortal. Which is why the Stone Table exists, and why Halloween is so important even to immortals - it allows them to gain power when they otherwise cannot. It allows
change. Yes, it can also result in their deaths or the deaths of their enemies, but I suspect it's far more important to try and steal power and add it to one's own. That's far safer, but also longer term thinking than a slugfest or assassination (both of which are more likely to fail and may even result in death to the attacker).
Hence, if one did attempt a Darkhallow on Demonreach (remembering the only time it can be attempted IS on Halloween given that a mortal cannot become immortal except during that type of convergence - which doesn't mean there are not other places e.g. specific locations in the Nevernever that one could do a Darkhallow), one might be able to consume the immortal beings if released.
BUT - the variables are too high I think to be feasible. You have to kill each immortal, as I can't imagine they would go willingly, and they would each put up a fight. They are likely well rested and angry. Imagine trying to face down 100 Ethniu-level threats. And Demonreach contains thousands of monsters, maybe more. While not all would be immortal necessarily (although likely most are, given that the Naagloshii are the least powerful threats and they are immortal), you still have a lot of really big threats to contend with.
Then there is the issue of what happens if you did somehow manage to weaken enough of them to consume them in the vortex. I doubt there would be anything left of the performer of the Darkhallow. Given Dresden is probably the only being who could both release the prisoners and perform the Darkhallow given his knowledge and position as Warden, I doubt there would be a Dresden at the end. As CT says, you are what you eat. Jim has spoken a bit about this before. In consuming so many powerful identities (let alone dark, insane monsters), it wouldn't leave much left of the original being that ate them. The act of consuming them is an act of change, and the only thing that can change is the performer of the Darkhallow. Sadly, what would be left is a truly insane, incredibly powerful monster. But I doubt it would ever get to that stage.
I don't think most of them would be subject to "psychic vacuum" of the ghosts being eaten.
Mortals are relatively-weak, with little in the way of defenses; not so the entities in the Well.
This is right, I think. In their containment crystals, I think they would be protected against most things. Not to mention most if not all of the beings in the well are immortals and so basically impossible to kill. Beyond that, the magic of the Darkhallow consumes all the magic in the area (I suspect because the Darkhallow uses necromancy, which uses the power of death rather than life). This consuming of magic (i.e. life energy) is what creates a vacuum, and reality balances that by ripping all the life out in the local area to bring local reality back into equilibrium. This energy doesn't go to the Darkhallow performer, as they already have consumed the areas magic, it's life energy. The vacuum is an after-effect, a very dangerous and scary one. But not evil in and of itself, no more than a hurricane is evil. Just nature balancing itself out. I also don't think spirits (including immortals here) are the right sort of energy, given that strictly speaking, they aren't exactly alive. They are energy beings, sure. But mortal life energy has repeatedly shown to have a qualitative difference to it. I don't think the energy of the immortals would balance things out even if it could be ripped from them, which it can't anyway.
That may be debatable - whose magic? Beyond human magic, apparently beyond even Ethnieu - but maybe a Mother could crack it, and I assume Uriel could. Kinda like bulletproof vests - depends on the bullet. That darkhallow is big power.
I don't think it was beyond Ethniu. She didn't know of it's significance. If she had, she might not have attacked so close to Chicago. Remember, He Who Walks Before was potentially able to crack it given enough time. Not to mention, it's not merely a matter of metaphysical/magical might. Mortal power is enough, given that Before was using mortals in it's rituals, and Ethniu had plenty of mortal servants too.
A being of the Mothers' level (including Uriel) could do it, but I doubt any would. Not to mention, even if they have the horsepower necessary, none of those beings seem to be able to influence events on the mortal plane except to the tiniest degree (although Lucifer seems to be able to get around this a bit more than the rest).
Anything powerful enough to destroy Demonreach would destroy the Earth as well.
Depends on your definition of destroy. If the Well's failsafe is activated it destroys most of North America. Such an event would be worse than any meteor to ever hit the planet (including the potential one theorized to have ended the Dinosaurs). Such an event would physically change Earth's climate to likely a completely inhospitable degree (given it would likely cover the planet in a dirt cloud that might not disperse for years). This assumes that the Banefire isn't so hot that it ignites the atmosphere, burning it away completely and leaving the Earth completely unprotected to the hostile nature of space. That would result in half the planet instantly burning and the other half freezing (depending on whether the light of the Sun was touching it), and this would be after the initial global firestorm. Now maybe if you were in a bunker deep enough you might survive all that, assuming you had your own oxygen supply etc. But not for long. The oceans would be evaporated, the entire surface made inimical to life. No air, water, safe and arable land, no food. Depending on the power of the Banefire's explosion, it might cause tectonic disturbances. Which means any bunker that wasn't immediately blown away, might still eventually become totally compromised due to the shifting continents.
That's a destroyed Earth by most definitions. And sure, there are lesser scenarios that only result in the USA getting firestormed on the surface. But that likely would still cause a massive geopolitical change, probably result in some very destructive wars, and probably still some negative climate effects on a global level etc. And this assumes no other supernatural involvement. Plenty of monsters might come out at this point to exploit the situation.
But if you mean a full-on Death Star-like planet-fully-destroyed-and-turned-to-rubble moment (overkill by most definitions), I think that's redundant.
Besides, why destroy Demonreach? Just as easy to leave it there and keep away any competition. Not to mention, simply cracking it open would be enough to end the world (in some cases even for just one of those beings getting out) let alone all of them. As I said earlier, cracking it open isn't simply a matter of brute strength. Skill, knowledge, will and tools are just as capable (perhaps more so in some cases). It might be easier and have a higher chance of success to simpy use leverage. Even the Outsiders didn't just use brute strength, and they have potentially an unlimited amount (by some definitions). They used a variety of approaches.
