Considering the little tweaks here and there to the lore that have occurred even during the EA, I'm not sure how seriously to take the teaser trailer that was made who knows how far into development. Because of the dreams we do know that the tadpole is interested in destroying Baldur's Gate, and the Sword Coast afterwards.
About our tadpoles being different from other True Souls, I'm not sure where you're getting this from, apart from every other True Soul we meet having been to Moonrise Towers, what else is there?
There's references to Netheril a few times in the EA, including a book on a ruined city, I don't know enough about FR lore to earnestly work it into a good theory.
