Load only the mods you actually need for the mission first. And do that always, starting game with 123s mods because you want to play two or three missions without having to restart the game is asking for such issues.

FYI issue still exists, it seems like once the savegame is bigger than a certain uknown treshold no "continue.Arma3Save" is generated and unless you rename any existing save to the "continue" one, you're forced to replay the mission from its very beginning. Lost many hours already as it happens so rarely it's hard to get used to it.

Yes, just check the video I added to the Additional Information section. AI is simulated correctly when player is not in vehicle controlled by AI or AI is not in that vehicle as driver/another passenger.