History is a tough one. There's no doubt it is a combination of imposed and composed. Unless you take the position that, since history is the story of human action, that all of it is composed by the actors who took part in [insert historical event here]. That might be a stretch, but I'm not sure. Certainly events DID happen, even if our understanding of them is obscured or incomplete. I think you're correct that much of history - especially the distant past, but probably some more recent events as well - are unsolvable. They happened, but how and why...?
Are you familiar with the book, "The Swerve"? It's the story of how the Roman poet Lucretius's famous poem about Epicurean philosophy was discovered and therefore saved during the Renaissance. Epicurus and Lucretius take the perspective that everything from the world around us to the actions we take are simply the product of atoms randomly swerving and interacting and that there is no meaning to anything, just the accident of atoms colliding and our attempt to make sense of it. I don't fully embrace that line of thinking, but there is something to the idea that we lack control over things despite our firm belief otherwise. In that sense, history would be entirely imposed, but I think that perspective misses a lot.
Regarding critical race theory, or more specifically, anti-racism, I think there is a lot to be desired. I've read some of Kendi's work and much of it doesn't even pass the sniff test. I agree with you that a Department of Anti-Racism is a horrible idea that is almost certain to make the problem of racism worse, not better. Policing speech is certain to fail. Kendi also argues that every policy is either racist or anti-racist, but that's laughably silly whether one wants to look at simple policies like traffic laws or complex ones like American foreign policy vis-a-vis China or Israel. Speaking of which, Kendi argues that the only solution to historical racism is current anti-racism. As it regards Israel specifically, this seems to me to be a troubling proposition. The Jewish people are perhaps the most maligned group in history, dating back to at least the Babylonian captivity. They have suffered for millennia, and yet I would argue that the state of Israel perpetrates crimes against the Palestinians. It seems Kendi's thinking would condone Israeli actions given that the Jewish people have suffered so much historical racism, and yet I highly doubt Kendi would actually take that position.
And of course, how many people today simply see the Jewish people as "white," regardless of their historical suffering and even as white supremacists in Charlottesville chat "Jew will not replace us." Whether or not history is imposed or composed, too many of us have basically no understanding of it.
Finally, my perspective on nationalism is that it is almost always toxic. The concept, as you said, lends itself to an us vs. them mindset, whether the us in question is a nation-state or a dispersed ethnic group. No doubt nationalism and the group identity it espouses are understandable and appeal to many, but nationalism forces us to see others as somehow different rather than as human beings. Ultimately that leads us nowhere good, even if the reason for embracing the common identity is a sense of shared suffering.