Concerning your sentence: "It cannot be a universal rule that one steal from the other, since one would be without and the other would have ill-gotten gains." If this is what Molyneux is saying, he can't know what he's talking about. This sentence assumes that it is bad for one to be without and for the other to have ill-gotten gains. He can't assume morality to prove a moral source theory. See what I'm saying? The gains are only "ill-gotten" if we accept a morality in the first place, in which case we don't need his theory at all.
Furthermore, from what you write, it sounds like his theory is nothing more than contractarianism, which fails because it's not universal. That is, if somebody could get away with an immoral act without bad consequences to themselves, in Molyneux's world it would be fine to do it without moral consequence.
I'm sticking with God.