Eh, I don't know about that. BG 3 is fairly dissimilar to M&BJoV. Aside from Sarevok, what exactly has held true between the two products? It might be that the author of the latter had an ear t what was going on and based his book off of that, but it doesn't mean that His book constrained Larian in any way, and in fact IIRC WoTC has said that the side products (games, novels etc) get much freer leeway with canon under their new policy.
I'm sure they handed down some dictates, vague and otherwise, but there seems to be leeway for interpretation on some things (see the Dark Alliance reboot with the 'Lolthsworn get magic tattoos' thing vs Larian's 'Lolthsworn get red eyes' thing. And I don't think there's any indication Larian was ever handed a sheet of paper things they had to include in BGIII that included 'rewrite fan-favorite returning character into a loathsome PoS that players will want to kill, and then make Shadowheart's good endings hinge on removing said PoS.'
