First of all, in the case of DoS2, when the npc was busy and the rest of the players did something inappropriate (not counting a direct attack), it started the fight. Sometimes it could just be an attempt to get past the guard. It was not used often, but it was present. So if Larian wanted to, this restriction is not a big deal.
There is probably no one in this forum who would say that the steath is not broken. This is largely an AI issue and I don't know if they can fix it in some way. It would require quite a smart AI which is rather rare. You could make the steath a bit more difficult. I think that would be enough to just add small circles around the enemies. At best, however, it will make sneaking slightly more difficult. As for armor sneaking, it is perfectly feasible even with full plate armor, the problem is that it is incredibly difficult and rather not very profitable.
Should characters that have not been detected automatically be involved in combat? Or should combat automatically trigger turn-based mode for the entire team? In my opinion no. It would limit the freedom of the player very much. At most, characters should only be drawn into combat automatically when they are connected to each other (it sometimes worked differently in the case of DoS) The strength of dos games is that players can do whatever they want independently of each other. Honestly, the more freedom you give the player, the more it is facilitated by various types of exploits. At the same time, if this is the basis of your games, it might not be wise to change it. The game is designed from the ground up as a multiplayer game after all.
