Yes, it is necessary that a thought experiment accurately reflect reality for it to be viable. Otherwise it doesn't have any meaning for reality.
Simply because no person exists to be immoral doesn't mean that there isn't a universal morality, either. It only goes to show that we are sensitive to the laws of morality, just as we are capable of being sensitive to atomic forces.
There are cosmological theories out there that claim that the universe will never enter into a state of chaos, because the cosmological constants keep the universe in a state that ensures it never will. Like a rubber band, that if stretched even the slightest it will inevitably return to it's normal shape. I believe the evidence for this is overwhelming, just notice the big bang. If the big bang were uncontrolled, the universe would be uniformly scattered hither and thither already, comprised mostly of empty space.
If a tree falls in the woods and no one is there to hear or see it, does it still fall? Did it happen?
Just because no one is there to observe morality doesn't mean it doesn't exist. And I have shown ontologically that it must exist. To say that universal morality does not exist is to be entirely ontologically egocentric to the point of claiming control over reality.
