I don't have time to respond to your question about why one should think that God's existence is metaphysically necessary, but I did want to respond to your question of whether God could have been different. If God is a metaphysically necessary being, then He could not be different than what He is. That's what it means to be metaphysically necessary. So if God could have been different in different worlds, He would not be a metaphysically necessary being.
silentmatt wrote: The best argument for God's metaphysical necessity, I think, comes in the form of the various ontological arguments based on God's properties. Some, like Alvin Plantinga, have argued that we have reason to believe in God's necessity on the grounds of his maximal potency, that is,since a being is more powerful the less his power depends on the circumstances that he finds himself in (that is, the more worlds there are in which he has his power), a being with the maximal possible amount of power must have his power in all possible worlds, in addition to having the familiar unlimited power in his particular possible world. Since a being cannot have power if it is nonexistent, such a being must exist in each possible world in which it has power, that is, all of them. So such a being, if he existed, would be necessary. If it is possible that such a being exists necessarily, and it certainly seems that it is, then necessarily, that being does exist. Therefore, such a being necessarily exists.