Is Diversity Flatly Good?

I think you're trying to justify your own bigotry.

There's not a lot of diversity in Afghanistan, pretty much everyone is a Muslim yet there's a ton of strife.

The America Civil War had a lot oif strife as well, and that was before 1965.

And I don't think the indigenous population would agree with your assessment 0f a strife free America prior to 1965.

The problem with a lack of diversity is that is creates holes, places where society is oblivious to its problems because it is so homogenous that it can't see them. I have spent some time in Japan. Japan is exceptionally homogeneous. I also found it noticeably racist. Many of the atrocities committed by the Japanese in WWII could be directly attributed to their view of their innate superiority to other peoples. (The same could be said for the Germans.) A lack of diversity can easily lead to a lack of shared humanity.

To your specific point:

This is completely not true. The increase in strife is pretty recent. Nixon played on the "Southern Strategy" to promote racial tensions for political gain, but all through the Clinton administration, the US was thriving. Crime was dropping, the economy was growing, racial and gender equity was on an upswing (although with far to go). I think there are three major events that drive our current divisive atmosphere.
1) The election of Barak Obama. Despite being a pretty moderate President, Obama's election really brought out the quiet minority of extreme racists. This gave rise to the "Tea Party" movement and radicalized some Republican districts to the point that moderates were driven out of the party.
2) The rise of social media/extremely free and easy communication. It used to be that if you were a racist nutcase, you pretty much stewed alone or maybe with your friends at the bar. With the explosion of the internet, you can very easily find others to support your worldview and recruit those who are impressionable. Besides racist communities, this has really been a boost to conspiracy nuts as well.
3) The commercialization of hate. Let's face it, there is big money in stoking hate. Fox News owes its entire business to stoking racial resentment. Many of the more radical communities out there are driven by just a few people with huge media presences. A well-spoken, intelligent, middle of the road opinion will get no traction at all, but a rant with ridiculous lies and distortions will be all over the place in hours generating clicks and revenue streams.

I suppose the battle has always been such, but right now, the table has tilted towards misinformation and hate, but I don't think it has anything to do with diversity.

You really need to question your own 'quotes' for accuracy . There may have been additional information you missed.