Sorry, you're exagarating the complication here, while that puzzle was already solved long ago in DOS2 for co-op. In that game when you take a backseat in co-op for certain important dialogues the game takes a little pause and takes away control from you to make your character come to the needed point.
D:OS2 didn’t have cinematic dialogues so it solved it by not having silly idea to cram cinematics into a multiplayer RPG, which is indeed a solution I would welcome.
Your suggestion could be interesting as long as you don’t think on it too hard. The game would need to track where positioning is in cutscenes, and move “actors” accordingly. From player perspective It sound doable, however, pathfinding barely works as it is. I expect that NPC would end up doing some weird shit, like getting themselves killed, setting off traps etc. Can you imagine a cutscene starting after the battle and game moving your party into the middle of burning oil to match the cutscene?
