Or hell is where humans placed him. They created the stories after all.
Although I know you might not agree, we do know from Jim that these spirits existed prior to mortals being created. We know that Angels and many Gods existed before time began, before Creation. So a lot of things happened before human belief had any impact. Rather, they learned the stories from others. The beliefs didn't come from nowhere. Jim once was asked if there had been multiple Creators given that human belief changes things, and so as belief changes the retroactively empowered different beings. Jim replied that the questioner was assigning limits where there aren't any - that it isn't so much the beings that change but mortal understanding of who and what they are that does.
So, to put that in context of your statement, Hell and Lucifer always existed, it's just mortal understanding of those things that change. Mortal belief doesn't really change that sort of thing. It just learns more about it.
First my standard disclaimer, it's your book when you read it.
I love Butcher's prose, but language is language. If a simple statement as in one archangel did one thing then another archangel did another, then it is pettifogging if he then tells me that it was only true in a way. I don't care who connected the hose to the fireplug, I am concerned with who supplied the water.
In terms of Hell, wherever it is, it isn't Heaven. They are called Fallen because they were cast out. And that the devil might rather reign in Hell than serve in Heaven doesn't obviate the fact that he wants to be is on the Golden Throne in Heaven.
I like your standard disclaimer. I always assume that of course, but it's nice to hear it all the same.
I agree, the supplier of the power is the issue. I think the likely explanation is that when he wrote Small Favor, he literally meant Lucifer supplied the power. I think he has revised that idea somewhat (for reasons that are unknown) and needs it to be a lieutenant. Based on what we have even seen or heard about "normal" angels, it is hardly beyond their power, in fact it is actually well inside their abilities. Given that Lucifer is an archangel, it's a fairly small move for him. I think the later text does hint that Lucifer might have other reasons for not being directly involved. I mean, if you go back far enough we are told the Fallen cannot act directly. Hence why the Denarians are around. But then we see some contradiction over the series and are all left wondering what happened.
I think that very much depends on the story. In some interpretations, Heaven is a totalitarian dictatorship, and Hell is the place of freedom. Now I doubt this is true of the Dresden Files but when it comes to fiction anything is possible. In as much as we know about the Fallen in the Dresden Files, they "chose" (as much as they can) to not be angels anymore. An angel isn't a species, it's an office. The "species" is spirit. I think when it comes to such beings though it affects their very being to occupy or vacate an office, unlike mortals, and so the spirit that was an angel becomes Fallen, a sort of anti-angel, a negative of the same being. Why that is so is unclear but my guess would be it is to do with the very nature of holiness and sin.
In any case, we don't know the story the Dresden Files has gone with. Perhaps the Fallen were cast out. Perhaps they left of their own accord and set up shop elsewhere. In some stories, like the Lucifer comic, the throne of Heaven is the last thing Lucifer wants. He doesn't want to rule Creation, he simply wishes to be free of it. I don't know what the Lucifer in the Dresden Files wants, all I know is that he is having an "argument" with The White God (the Creator and ruler of the Dresden Files Creation/multiverse). What the nature of that argument is, what the positions are, and what that looks like is mostly unclear, but we do know it has something to do with influencing mortals to choose one way or another. Ultimately, we cannot assume that the Dresden Files' version of Lucifer, or the Fall, or the War in Heaven, is exactly like that of the Bible. There are already enough differences that we must be open-minded to other possibilities.