I've only gone into MSQ roulette a few times, but it should be done with the understanding that giving new players a full experience of the dungeon is the #1 importance. Getting the whole story and pacing, and seeing your own character there in the cutscenes is always going to be better than watching someone else on Youtube.

Perhaps the tomestone reward needs to be given only if you're standing near the new person when they get to the end of the cutscene....? Or just lock everyone's movements, like the circle at the start of dungeons prevents people from moving if someone is watching the opening cutscene.