Hopefully this doesn't surprise you, but I agree with 90% or more of your response. I don't claim to understand Nigeria after only spending a few months there, but the situation did and does fascinate me. I wasn't offering the America way as the solution, simply presenting how our nation wrestled with deep ethnic divides and at least relieved enough tension to avoid what could have been a much bloodier civil conflict.

True, the story of African Americans was sad, but there was a common understanding of what an ideal America should look like
.

You touched on a key point. We did have an idea of what being American was, it wasn't about skin color or gender, but it took us years to actualize the idea (at least get closer to it). That idealism is what enabled the Civil Rights Movement to be successful. If the idea of equality didn't exist it would have failed before it started, so like all things context matters.

I also think many Americans, myself included don't see it as a black and white issue, though that seems to dominate the media. To understand multicultural integration you also have to look at the Cubans, Hispanics, Russians, Italians, Chinese, Filipinos, Vietnamese, Irish, and a growing Muslim population among others. Yes it very different situation in Nigeria and Africa as a whole, but still worth studying.

You may be right that,
Managing diversity in the US is about tinkering around the edges of the dominant settler culture to make allowances for new entrants.
That is a perspective I haven't heard before, but it makes sense.

I don't agree with you that the US is secular. You wrote,
Did the US ever have to deal with a religiously derived legal system competing with a secular legal system?
We have been dealing with this issue from the beginning and continue to do so today. Many Europeans are surprised at the level of influence religion has on our society. In general these are not violent contests, but arguments contested in court. What seems to be changing is the strength of the far left and their political correctness movement which is similar to Mao's Reformation minus the millions killed, who are attempting to dictate through law what people can say, read, and think. In turn this is spawning a revival of the far right, and unfortunately the middle is disappearing. This seems to be happening throughout much of the West, so we're seeing a growing divide between the far left and right now that could result in considerable violence if we lose touch with our core accepted values that define us. Again we're not Nigeria, but we're not immune to these problems.

Finding answers for Nigeria is the task of the Nigerian people. We need to focus more on solving our own problems. Of course the West and UN could intervene and attempt to impose a solution on Nigeria if eventually comes close to failing, but that is almost guaranteed to fail before it starts.