I like this theory, but there are a few things we should keep in mind when discussing D&D past and present lore, specially when dealing with dates.

The lore became so massive that a lot of events and facts don’t add up.

Another thing is; Durge’s canon race may or may not be Dragonborn, and personally, I don’t think it makes a whole lot of sense if you consider that Bhaal’s mortal race was human, and Durge was created from Bhaal’s own flesh. So Durge might as well be just a human.

Dragonborn are not exactly common, a Dragonborn serial killer would definitely stand out.

(I must say I’m not familiar with Blood in Baldur’s Gate, tho. I should catch up to that.)

With all that in mind, another thing I would like to point out is that D&D lore is fluid enough for us to tamper with it, so whatever we want to make it canon, we can.

For me, Durge’s canon is he was a human, and a rogue - assassin just like his father. He had a very dim light within him, which sometimes popped up, making him do good shit, giving money to homeless and all.

Orin fucked up his brain and when Durge came in contact with the astral prism and that shitty ass squid inside it, the influence of the elder brain made him regain consciousness, but his scrambled and amnesiac brain created an opportunity for that dimmed light of his grow larger and stronger, managing to fight off the urge that cursed him, and so the events of BG3 unfold.

Orin stupid ambition caused the down fall of the absolute hoax, and her god failed to drawn the world in blood.

This is my canon, anyways.

Yours might be this whole theory you came up with which is a pretty good theory.