Welcome to the forums, Fillmoreb.
It's perfectly fine to bring up previously discussed topics, lawd knows we all do. Everything here has been discussed and dissected to death, then resurrected and the process starts all over again!
I have thought about this a lot, but sadly I think this is one of those you can't see the nose on your face issues. Jim said it to mess with the fans, and I suspect it will come true in ways unlooked for.
I like your ideas. I don't think I am convinced about Demonreach - Bob and he don't seem to acknowledge each other any more than both being spirits. Lea is a possible suspect, but there are reasons she also doesn't work. We will get to that. Leah is technically a creature of both the spirit and mortal world...but it's tricky too. Anyway, I will give you my thoughts.
I am almost dead certain Bob's mother is Mab. Considering his entire deal around her...it would make sense. Also given some of the information he knows, Spirits of Intellect gain their knowledge from their parents, at least initially. Not to mention, he is a spirit of
air. What is Mab the Queen of? Air and Darkness. And Bob has a
very dark side. A blue side, I might add. Also, Mab is known for logic and reasoning...and Bob is a Spirit of
Intellect which counts for the same.
Now, considering Mab's great love was Merlin (this is clear from Peace Talks) we could be forgiven for assuming they are both the parents.
But wait! Are we not supposed to have met both parents, and we haven't met the original Merlin Emrys (whom Corb says is not "among the living")?
But as many characters (including Mab and Lasciel) have pointed out, death isn't so black and white. Perhaps we did meet Merlin, but under another name and guise...
But there are other possibilities. Jim has also said SoI's are made of the combination of an immortal and a mortal parent. But as Harry has shown, that can be two beings in the same body.
So it could have been Mab while mortal, with an immortal in her head. Or Merlin for that matter. But what immortal would that be?
Not to mention, there are many, many side stories. Not even all of them from when Dresden was alive. So it could well be something not in the main series at all.
I did wonder about Kemmler for a while, but he only got Bob well after his creation. Vadderung seems a good choice, he had many sons. But why wouldn't he give Bob a body (gods can do that, that's how Athena came to be from Zeus)? Could Merlin have just been a guise of Vadderung?
Rashid isn't a bad choice, Mab seems to have a special interest (that we are meant to assume is due to his role) but still. And he is old enough, technically. He also seems to have taken his role as Gatekeeper right around the time Mab ascended to Winter Queen.
Another option is the Archive and someone/thing. Because Bob is basically just a little Archive, without the body. But then, we still don't have the other parent.
It's also worth considering Bob's nature. He is perpetually horny, a wiseass, a bit of a coward at times, and a very, very powerful psychic demon. Considering the sex-focussed spirits, I can only really think of the White Court being involved. That being said, a lot of those personality traits are to do with being picked up by Dresden at age sixteen.
So, what's the answer? Who knows. But in order for me:
1. Mab (Winter Queen) + Rashid
2. Mab (Winter Queen) on her own
3. Mab and the OG Merlin (although he then has to have a different alias we have met somewhere in one of the books, even a short story or comic).
4. The Archive + someone
5. Vadderung + someone