Paul does not claim that Creation leads cultures to general monotheism. Indeed, Paul describes many as "changing the truth of God into a lie, and [worshipping] and [serving] the creature more than the Creator" [Rom. 1:25]. The claim is not that cultures are driven to monotheism or there exists a significant cultural awareness. Rather, the argument is that the information is available to individuals, but is generally rejected.
Nor is Paul claiming that cultures gravitate towards a Christian ethical understanding. Rather, he is saying that a general sense of morality is given to all men which they and their cultures sometimes follow. For example, when a person or a culture recognizes murder as wrong, they do by nature the things in the Law as a result of conscience. I would expect a study of cultural morals to reveal similarities in some morals to those presented in the Law, just as we have already observed religious similarities across many religions. Paul attributes these similarities to a universal conscience given to all people, providing a general moral understanding and conviction of our own moral imperfections.
Burden of Proof for General Revelation
As a result, your burden of proof is excessive. First, it exaggerates my claims from personal understanding to cultural ideas. Second, our knowledge of ancient and isolated cultures is limited at best. This makes it hard enough to know what they actually believed, let alone, what ideas they may have been aware of, but rejected.
Revelation of Christ
Regarding the propagation of the final principle of salvation I am aware of two views. First, anyone who accepts the 2 principles of general revelation will be sent a human messenger to address that question (perhaps indirectly, by making literature available or directly by word of mouth). Examples include the sending of Philip the Evangelist to the Ethiopian Eunuch [Acts 8] and the sending of Peter to Cornelius [Acts 10]. If this is true, the fact that the gospel was not sent to a Native American in AD 600 is considered proof that no Native American accepted general revelation. This is reasonable and hard to contradict. The second view observes that there is no passage in scripture in which God explicitly states He will not provide special revelation to those unreachable by the common method outlined above. After all, Jesus Himself intervened in the salvation of Paul (Acts 9) and John mentions the inner light given to every person.
Your argument seems to be that it is likely some Native American accepted general revelation and was denied the final principle for spatial reasons. As a result, you conclude that Christianity violates the salvation ubiquity criteria and must be rejected as a valid world view. At best, the existence of this Native American is highly hypothetical and rejects without substantiation the possibility of special revelation for that individual.
All of the above remarks substantiate my claims. Creation and Conscience are available to all men and God is more than capable of providing the specific revelation of Christ to anyone accepting the first two. A detailed study of isolated cultures is unnecessary and would be inconclusive at best given our spotty knowledge of such cultures. Christianity clearly provides mechanisms for meeting the salvation ubiquity principle. That does not make it true, but it does make it a reasonable possibility given our discussion so far. Furthermore, the salvation ubiquity principle provides a clear example of a criteria that can rationally evaluate religions, accepting some and rejecting others, providing hope that the morass of religions is not as un-navigable as Atheist Under Ur Bed suggested in the article I originally responded to or James suggested in his initial comment on my initial post.