If a necessary being is impossible - you can say there are possible worlds without God. If God exists, then no possible world can exist without him.
Once again, please define what you mean by the term 'possible world'.
For example, why is a world that has only a piece of cheese not possible?
So if the ontology of him being necessary proves he exists, it's also proven no possible world can exist without him.
And if the ontology of a magon proves such exist, then no possible world can exist without magons.
And this should be easy for people of who are of Abrahamic faiths to believe. We don't believe it just happens to be there is One God. If there were possible worlds without God, there can exist gods aside from God.
But we believe his existence doesn't allow that. It's not just that gods don't exist aside from God, it's that it's impossible. The same arguments for one God are actually rephrasing the ontological argument with respect to gods existing. But with thought of with respect to existence, it shows he exists definitely and is the Necessary truth by which all truths including moral and logical truths are found upon.
Yes, you *believe* this. But you have not offered a proof for these beliefs. You reject them for no reason other than your previous assumptions, which you have not justified.