Whether you consider this argument to be intended to inform rather than persuade is irrelevant (and ill-defined). Science, which proposes definitions to describe and explain observed phenomena, does not invent entities arbitrarily out of whole cloth and then attempt to reason them into existence; quite the opposite, in fact.

Your argument does not deduce that an MGB must exist in all possible worlds if it does exist at all, it defines an MGB as an entity that exists in all possible worlds.

It also does not deduce that it is possible for an MGB to exist, it claims it is possible as a premise. There is no deduction necessary to arrive at the conclusion, because the premise is the conclusion. The fact that you bury the definition halfway through the argument does not change this fact. See Gale (1993) for a fuller treatment of this. I am not the first one to point out this fallacy.

Even if you formulate the premises more rigorously, the argument still fails, because the premise "It is possible that an MGB exists" is completely unjustified. Consider some definition of MGB (omnipotent, omniscient, morally perfect, or not, it doesn't matter), and these premises:

If an MGB exists, it must exist in all possible worlds.
It is possible that an MGB does not exist.

By 2, there is at least one possible world in which an MGB does not exist. By 1, if it does not exist in at least one possible world, then it does not exist at all. Hence, we have proved that there is no MGB.

What justification do you have for taking "It is possible for an MGB to exist" as a premise, rather than 2 above? What happens if you accept both premises (they are not mutually exclusive, at face value)?

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