Larian didn't just write Durge's story out of nothing,
I believe it was supposed to be basically a mirror of Gorion's ward's self-discovery (as in Tav probably wouldn't know what they were at first), then I am betting someone at Larian got worried that it would be straight up copying of the original BG story bits, as the Slayer model has been in EA code since the very beginning. And the rumor that one of the companions would be a Bhaalspawn (which I think was a red-herring for Tav being the one). Then, when someone decided to disentangle Tav from the Bhaal plot line, making Tav essentially 'just a dude', and since they already had a bunch of story beats and the slayer form in the code, Durge was born. And they made it extra spicy and bloody to boot, to distinguish it from Tav.
At least that's where the EA datamines were leading to. Obviously Orin was always Bhaal's chosen, but I don't think Tav was originally the mastermind behind the Crown heist.
How am I so sure? Well, interacting with a bunch NPCs as Durge feels really... tacked on, especially the NPCs Durge was supposed to have known pre-amniesia.
Also, basic Tav is the only one who doesn't get to meet avatars of all the Dead 3 representatives. Myrkul we meet at the end of act 2, Bane - speak with dead on Gortash's body and Bhaal is exclusive to Durge only, suspiciously missing from Tav. You'd think he'd make an appearance to gloat at least, or threaten.
