Perhaps a philosopher of science can address this question more cogently, but I'll make a few observations. You seem to be assuming that the theory of the ancient greek philosopher and the modern theory confirmed by contemporary experiments are the same theory, or posit the same entities, say. But it seems likely that the atomic theory of Epicurus or Democritus, say, is not the same theory of 20th Century physics, just as the ancient greek methods and those that Lord Rutherford used to confirm the atomic theory were also dissimilar. So I don't see how those ancient thinkers could have 'earned credit' for a theory they themselves did not propose. Nevertheless, we can still recognize the prescient nature of their insight into reality and refer to them as forerunners or precursors, if we wish. As for the difference between lucky speculation and insight, I am not sure that's a useful distinction to make.

If Hegel relies on false observations about other cultures or false historical claims to make his arguments in the philosophy of history, then I would think that it would undermine those arguments. I suspect that those false claims do in fact serve as evidence for his views in the philosophy of history, which means that his views are indeed undermined. But perhaps his false and distasteful historical and cultural claims are not meant as evidence for his theories, but illustrations of them. If so, then his claims are *again* weakened by his errors. For, if his theory makes predictions for phenomena that do not in fact obtain, then that would seem to suggest his theory is not satisfactory. Finally, if his falsehoods are neither supporting his theories or supposed to stand as examples of the explanations those theories provide, then I suppose they would not weigh on his views, casting a bad light merely on his character. But surely other readers of Hegel may disagree.

I shall address your question concerning reason and the law of nature and shall leave aside the last two sentences concerning morality. It is true that Locke believes that reason allows those even in a state of nature to know the laws of nature and, therefore, to follow them. Thus, in principle, one might think that people could live in harmony in a state of nature, guided by reason alone. Unfortunately, when people live together or interact with other, which we must necessarily do, disagreements will sometimes arise. What's more, when someone is a party to a dispute, they are not always able to reason properly concerning the dispute. This is a fact of human nature; we are not always good reasoners when our interests are involved. In order to adjudicate such disputes, we need a third party to help us. Thus, because people will inevitably come into contract or property disputes, and because they cannot always adjudicate their own or their neighbors' and friends' disputes rationally, we must...

Blaise Pascal, though not a medieval philosopher but an early modern one, did recognize that his famous wager was not always sufficient to cause an unbeliever to believe. Thus he recommends spending time with believers, performing the rituals with them, and, eventually, one would come to believe. He says: "You would like to attain faith and do not know the way; you would like to cure yourself of unbelief and ask the remedy for it. Learn of those who have been bound like you, and who now stake all their possessions. These are people who know the way which you would follow, and who are cured of an ill of which you would be cured. Follow the way by which they began; by acting as if they believed, taking the holy water, having masses said, etc. Even this will naturally make you believe, and deaden your acuteness. "But this is what I am afraid of." And why? What have you to lose?" (Pascal, Blaise, 1670, Pensées , translated by W. F. Trotter, London: Dent, 1910; #233)

To understand what people might mean when they say that, if God exists, God need not necessarily be perfect, infinite, unitary, etc... it helps to consider which arguments in favor of God's existence are under discussion. For example, consider the teleological argument which is, roughly, that the universe, or its creatures, have characteristics that could not have arisen through chance, or through evolution via natural selection. This argument sometimes takes the form of an analogy, and other times as an inference to the best explanation. In either case, though, the defender of this argument concludes, the universe and its creatures were most likely endowed with these characteristics by a creator. Given the nature of such a task of creation, this creator would need to be great indeed. Now, this reason for believing in God -- because the universe and your own body, say, are too miraculous not to have a divine creator -- only requires a God capable of such creation. But such an act of creation...