Ivy, I'll respond point by point instead of using the quote boxes, because after a certain point the boxes get confusing.

1. Mark of Cain. Ancient (pre-Talmudic) Judaism did not teach exactly what the Mark of Cain was, so what is the specific source for this claim, that the Mark was black skin? If this is a teaching that goes back to the early church and skips over the "apostate" church era, shouldn't it have existed in ancient Judaism?

Moreover, it's not knee-jerk political correctness if it is something that really is wrong. If it didn't exist in the Tanakh but really was an idea created by Joseph Smith and his followers in the zeitgeist of early American racism, then why would it not be wrong to condemn it? It clearly associates skin color with evil, and that is racism at its core.

2. "Less valiant." If this was a teaching held by most, if not all, of Mormonisms early leaders -- particularly its founders -- how can it be said that it was not AT THAT TIME accepted by the LDS Church? In other words, how was this not officially accepted doctrine pre-1978?

3. Pre-existence/polytheism. Mormons have a right to believe in those polytheistic doctrines, but in doing so how is it proper for modern Mormons to call themselves Christian when Christianity is and always has been monotheistic? Why not just state that Mormonism is a different religion?

4. Early LDS leaders and racist statements. It's not cherry-picking. It would be cherry-picking if I was selectively picking out statements favorable to my position. Yet I encouraged the reader to do his or her own searches to see that the statements are fairly uniform. I also do not have a side in this debate nor any personal desire to assert that early Mormon leaders were racist or were not racist. It's simply a fact that most white Americans pre-1860 had a range of anti-black prejudice. Early Mormon leaders shared that worldview. My ancestors who lived during that time period probably had those same racist beliefs. I'm just stating facts.
The example you gave from Jacob 3 is fine as an example of a verse that could be seen as antiracist, but that does nothing to change the existence of the many statements made by early leaders and in other verses. I know you believe that Smith and Young were prophets, but even prophets make mistakes and at times believe things they shouldn't. Moses was a prophet yet he was condemned when he did things he shouldn't have. King David was a prophet...same thing. So why not just say, "Yes, some Smith and Young said some things that were racist and they were wrong and shouldn't have said them. When they did right I praise them and when they did wrong I condemn them"?