The answer is perfectly clear in scripture. God never changes; He is immutable. However, his mandates and laws DO change. You can argue that certain laws were given only to the Jews, but even those laws were shown to be outdated and changed even for the Jews. "What I have called clean..."
He also changes his mind. Refer to the book of Jonah. Also refer to the total destruction of Sodom and Gemmorah, when Abraham talked him into sparing the cities if He found enough righteous.
So, His law changes, His mind changes, and thankfully His methods of dealing with humanity changes, or many more countries would have been destroyed since the OT.
You have even admitted that God changed his rules, when according to you He extended the rule to include half-siblings. Guess what? Extending the coverage of a rule is a CHANGE. You can't get around your own logic.
What doesn't change is His nature. He is perfect, and perfectly just on a level we can't comprehend.
The ONLY answer that doesn't totally render the Gospel a complete lie involves what we now would call an incestual relationship.
You have tried and failed to get around Eve being the "Mother of all living". If there were any other people created, she wouldn't be their mother. Period.
Simply put, you are adding to scripture by saying God condemned something.
Not only is your view unscriptural, but it is heresy, as it would render the Gospel a lie.
Now let's look at your "like begets like" argument. Even if God did create different races in the beginning, I think it's safe to assume that Noah and his sons were the same race. So, where did the different races come from, using your logic? Your logic falls apart at this point. Even if you argue that all three of Noah's sons married three different races of women, that only accounts for three races, and they'd all be half the same race. Unless you keep adding to scripture and say God had a second creation at that point.
Your position does not stand up to scripture or to intellect. I suggest you take a hard look at it and simply accept the only possible answer.