Why can't there be complete nonexistent objects? Imagine the God ofI say: if the God of Leibniz is contemplating something, then there is something he is contemplating. And I say that if each of them is determinate down to the last detail, some things are equivalent to them. And if each of them is complete, at least one of them is complete. All of the consequents imply existential statements, and whatever follows from the consequent, follows from the antecedent. I may be wrong, but all of this looks like an elementary example of the quantifier shift fallacy. If it is possible that a unicorn exists, it does not follow that some unicorn is such that it possibly exists. 'Possibly Ex Fx' does not imply 'Ex possibly Fx'.
Leibniz, before the creation, contemplating an infinity of possible worlds, each
of them determinate down to the last detail. None of them exists or is
actual. But each of them is complete. One of them God calls
'Charley.' God says, Fiat Charley! And Charley exists. It is exactly
the same world which 'before' was merely possible, only 'now' it is actual.
The very last argument (that the possible world is identical in all respects, save actuality, to the world that actualises it) is similar to an argument that Burley considers in his Questions on the Perihermenias. I will dig it out later, meanwhile I have an old comment on it here.