I agree with OP about excessive advantage and implictaions. I redid one combat were I started with height advantage, and it was vastly easier. Also, the AI always scrambles to get all your opponents on to high ground, precisely because advantage is such a huge deal in 5E. I also think backstab/sneak attack should work as intended - they already have a threatened status so the data is all there. The term 'backstab' was dropped from D&D ages ago, because well, you didn't need to literally 'stab someone in the back' ;-) It really is a defining rogue feature and should be triggered more easily (as per 5e rules) - that was by design. I really hope Larian take this feedback on board - I am happy to see that cantrips no longer create elemental surfaces.
Now as for the Backstab ability I wouldn't want this to be taken from the rogue or the rogue/assassin due to the fact that one of their primary traits is stealth and so if you sneak behind someone with a dagger you should in fact get bonuses to hit and damage. After all, it's not every day that you expect someone to come up behind you and stab you in the back
