I was watching a playthrough from 2 years ago and at the end I saw something that kind of confirmed my suspicions.
When you're on the boat on the way to Grymforge (Grymforge wasn't even implemented at the time) you got the narrator giving you a rundown of the stuff you did up until that point and the last phrase she says is:
My suspicion was that Tav and DU were always meant to be one thing but at some point in development they made them 2 separate things. Maybe it was because the DU was too dark for some people or because they thought losing control of your character at some points was going to make some people unhappy or what, but it seems to be the case.
Image from ChristopherOdd playthough.