They kinda explain the party member bit.
spoilers for BG3 and kotor, kinda.
It's partly explained by Wyll in that he mentions he was incredibly powerful before the Nautiloid. I guess getting a monster fetus drilled into your brain is bad for it.
And they added Dark Urge to give the pc a storyline to discover. It's clear that the Dark Urge storyline was developed alongside the rest of the game, almost like it was supposed to be the default story. I guess they didn't want to do a Darth Revan so they made it a separate option.
I hope I didn't spoil things for you, I'm guessing from your description you haven't fully finished the game yet?