I understand your points, but, respectfully (as MBM would say), they're starting from a lot of flawed assumptions that undermine you. First of all, when you refer to the "West African slave trade," you appear to mean the trade as practiced by West Africans, without paying any mind to the European contribution to the slave trade. That, I guess, is a matter of perspective, but your big problem is this: while Africans captured and sold other Africans, their role in the slave trade is not the reason American blacks are so different from continental Africans. If you notice, there are many populations in the diaspora where the cultures largely retain their African character -- like Haitians, Afro-Brazilians, Afro-Peruvians, and several others.
The African-born ancestors of these groups were kidnapped and marched to the coast just like our ancestors were, and eventually shackled to slave ships just like us. The ENTIRE reason why some diasporic blacks are less "African," culturally, than others stems from what happened in each country after the Middle Passage, not before it. That, in turn, is a reflection of the differences in how enslavement was actually practiced in each new world countries. So while it may appear "chic" and sophisticated to blame African slavers instead of Europeans for what happened to us, you have to be a bit more discernful as to what you apply that blame to.
The second flaw has to do with the connection you're making between this ancestry issue and Kwanzaa. Let me first put out there, that like you, I don't care for Kwanzaa, but for different reasons. That said, though, your beef with Kwanzaa is misplaced, because as u know, Kwanzaa was invented by an African-American, for African-Americans, and does not really have its roots in Africa, despite its name and its elements. So assuming you were correct in the first place about blaming West Africans for our "American-ness," you'd still be wrong to use that to disparage Kwanzaa. If Kwanzaa existed as a celebration that existed in West Africa, then your issue would make sense (again, if we disregard my point above about the slave trade). As it is, Kwanzaa is part of an effort to "reclaim" the Africanness that was taken from us. But it reflects its American origin in many ways (being artificial, like so many things American; being rooted in Swahili instead of a West African language, reflecting Americans' lack of knowledge of geography).
Think about it though: there was no such thing as the Latino before either. But the Latinos don't have a problem acknowledging the Castilian roots of a lot of their culture (and many of them also acknowledge the African as well). They are not quite Castilian anymore, like you say we aren't quite "African" anymore. But that doesn't mean we can't acknowledge our African roots. Whether we choose to do that through Kwanzaa or other ways, the important thing is that we do it.