Honestly when I read this post.
I feel like they just ran out of time and told some coders to scatter books all over the place to fill in some gaps. Almost all of this stuff should be shown ON SCREEN. There are some fantastic flashback scenes for Shadowheart's backstory, why is this never utilized in the main plot? To further elaborate, in 90 hours I had like maybe 5 Dark Urge story scenes, that comes down to 18 hours between each progression. And in my playthrough it was the same for the main story tbh, I feel like I have barely seen Gortash, Orin, Ketheric or the Netherbrain on screen. They need to be more present in the story. The game just seems to keep sidetracking me into sidequests that never tie into the main story.
I think the ideal rewrite would be to be able to remove the tadpole by the end of act 1 (start of act 2) and by that time we should already have gotten content that sufficiently invests us into whatever the main plot is.
Many points brought up in the OP is stuff I have also said elsewhere so no need to comment on that. But let me just point out again that this game can STILL be the genre defining game it had the potential to be.