Third: still waiting to see the reasons why you think more hp less ac makes the game harder
This is easy.
It's because the player's attacks and spells are largely still using the same numbers as the standard 5e system, and that system has different HP and AC for the monsters ( NOT the PC's) than BG 3 does. Spells which affect a certain amount of HP are intended to take out multiple weak foes. Good luck with Sleep, Color Spray, and Burning Hands in this game. Good luck later on with AoE spells like Fireball, which are supposed to be strong against masses of low HP foes, which no longer exist.
Spells which rely on enemy saving throws are comparatively weaker because the enemy ability scores have not been tampered with. Sacred Flame has a lower chance of impact than Firebolt because of the AC change, but it does damage still based on the standard 5e rules, and that does not account for the increased enemy HP, so when you do hit, it is not as impactful as it is intended to be.