The biggest thing here is figuring out the demon. The rules are what she decided, but they are not written in stone. Figuring out a way to fight within them is still playing the way she wants them to. Ultimate you have to figure out what rules the demon will enforce, and which ones are bluffs. I don't think her pride will let her kill the MC, and she doesn't want the MC to know about the girls being threatened or that they will die. So her actions are shielding him, hell we don't even know if she will or even can kill them for sure. At this stage you have to try to call a bluff. And I think the bluff to call is that she will not kill them in front of the MC. She has threatened and scared them to intimidate them away from him, but I am actually not convinced she will actually kill the losers even, at least not right away. To do so the MC would notice. I think they have to take a chance and tell the MC because he is the key to all of this.
The other thing is the basis of this is his wish, His wish which was for a harem. Him getting the winning girl and the rest dyeing, this isn't granting that. The demon is monkey pawing his wish, but the rules of the wish, and her rules are not one in the same, that she could change the rules midway shows this. The spell was already woven and she cant stop it, so shouldn't be able to change it.