First, Marx was a radical in early 1840s. He exposed his belief about the necessity of a proletarian revolution in 1844 and he was an atheist from years before. No way he could become acquainted to the slavocracy elites or pro-slavery. I would love a not-anti-religion Marx but I fear that in 1843 is simply too late.
Second, unfortunately Marx shared some typical 1800s racial points, as thinking blacks were more "animal" then whites (it seems he was not happy after his daughter married a French mixed man, probably that could be the cause) and the Mexicans were "lazy". Nevertheless this never influenced his political philosophy, where he was always a strong opponent of slavery, and he expressed this points only in some private letters.
But he thought better about native americans: he and Engels elaborated their "primitive communism" conception reading Lewis Henry Morgan's description of Iroquois communities. So probably, although his dream will be always a revolution of industrial workers, he could easily see Comamche and Navajo tribes as part of original primitive communism.
At the end I think he will end as Carl Adolph Douai (
https://en.m.wikipedia.org/wiki/Dr._Carl_Adolph_Douai), with which he could partner, maybe more successfully (a German free state in western Texas was a Douai's idea, Marx could rally Germans and Natives in support and break away. Also Douai was a founding member of Socialist Workingmen Party, so maybe Marx will be able to introduce Marxism in American political spectrum). At the end he will be remembered as the John Brown's philosopher, a sort of abolitionist Martin Luther, while Engels will be the more industrial workers focused writer.