We saw how difficult it was to get Ethniu into the Well.  We saw how difficult it was for Harry to fight Shagnasty… how did the inmates on Demonreach get there?  Who put them there?  When did they get there?

Before you go down  this road consider what purpose it serves in the narrative. It obviously is meant to serve as a power source. Rashid had hinted at that. Otherwise there is no  reason for the place to exist.  It's too dangerous.

Also consider that Demonreach lied to Bob when he reveled what and who built it.  You  should know by now that Demonreach is concealing something.

Odin had something to do with it. Interesting maybe.  Does Rashid wear Odin's eye? Interesting question  2. Who were LTW and the River Shoulders so frightened of that they wouldn't tell Harry about Drakul and Starborn?

There is a WOJ on this.  It's gone.  Water would have washed away the magic. In any case as of Battleground there is at least one and possibly 2 other versions in the books.

I stared at her for a minute. Then I said, in a very quiet voice, “I always know when I’m being tempted to do something very, very wrong. I start sentences with phrases like, ‘I would never, ever do this—but.’ Or ‘I know this is wrong but.’ It’s the but that tips you off.”

I said, quietly. “And I feel like an utter bastard for asking this of you, grasshopper. But I don’t have anyone else to ask.”

Butcher, Jim. Ghost Story (The Dresden Files, Book 13) (p. 537). Penguin Publishing Group. Kindle Edition.

It could be a reference to Shiva or the Thor in Marvel.  It's at least in the realm of the possible that we seen glances of him at various times. In particular when the Curse in triggered in Changes and Harry looses two minutes. Also during his berserk rages such as when Murphy is killed and he uses lightning.

His eyes were an unsettling color, somewhere between dark honey and poison ivy, and the sharp angularity of his nose made me think of a bird of prey.

Butcher, Jim. Ghost Story (The Dresden Files, Book 13) (p. 78). Penguin Publishing Group. Kindle Edition.

The books tell you how summoning got out to the general public. A plan by the White Council to weaken the process by having everyone trying to do it.(Maybe White Knight or possibly earlier, I forget) 

Harry would have learned his lessons from Bob about demons and summoning. Also Bock's Books.

In the end all roads lead to Nemesis in turns of bad things happening. He manipulates groups to achieve chaos.

MacFinn's circle could have been no older then Chicago unless it was in some way portable. That could mean the wizard who built it is still around and possibly corrupted by  Nemesis. You can infer this because MacFinn wouldn't have wanted to share knowledge of what he was. Since he wasn't immortal it had to be done by a modern Wizard.

"The Council knew that you were the son of Margaret LeFay. They knew that she was one of the wizards who had turned the Council's own laws against it. She was guilty of violating the First Law, among others, and she had… unsavory associations with various entities of dubious reputation. The Wardens were under orders to arrest her on sight. She'd have been tried and executed in moments when she was brought before the Council."

"I was told she died in childbirth," I said.

"She did," Ebenezar confirmed. "I don't know why, but for some reason she turned away from her previous associates—including Justin DuMorne. After that, nowhere was safe for her. She ran from her former allies and from the Wardens for perhaps two years. And she ran from me. I had my orders regarding her as well."