Like whats the goal, to make the harem accept that he died?
They wont accept it cause he is alive and has all his memories anyway. Like Nia just agreed to stay on earth(and she wants to eat him)
Oh, MC has already made it clear that he wants to live a peaceful life in that world. Whether or not he succumbs to his "sister" trying to seduce him is another matter entirely, and Gremory will be happy enough just to be able to go back to her home.
As for the Harem, each one of them wants the MC to themselves (though Nia's affections go beyond having an Electra Complex), so they're all in a low-key fight to be the one to get there first. And by "there" I mean being the last one standing. And it seems to me that one tactic is to put the others into a dangerous enough situation that they might get killed. Which can happen pretty easily when you rely so much on magic and are then put into a world with very little magic and highly advanced technology that you don't understand.
The end result is probably going to be that they all end up on Earth, living like normal people do, except the battle to be the MC's one-and-only will continue ever-after. (Why, hello there, Status-Quo-Is-God Trope!)