4) No one has really gotten that far enough into the game to know, besides the few special people hand picked to take part in closed beta of the final two chapters. I cannot give a fair assessment as I am playing on normal difficulty either, but most people seem to agree that Core and above is bullshit and poorly balanced. It's largely going to depend on what you're really looking for in this game.
I do not know who do you refer to with "most people" (¿?) There is even a thread asking for a "harder than unfair" mode via mods.
Anyway, I have experience in Hard mode and it´s... hard. But you have the possibility to customize the difficulty options. You cannot hit anything or enemies just save everything, lower the stat growth option, if you feel there are too many enemies, lower the difficulty option, if you feel the progression is too low... well. you know.
I really wish for Bg3 to have an option to lower the hp bloat of goblins, or the damage of fire for a low level party. Maybe we could have it before game release, one could hope...
