There's not really a debate about whether or not slavery was the cause of the civil war, at least not among historians. You can go a lot deeper on this if you want, but take for example one of the first sentences in the Mississippi Declaration of Secession: "Our position is thoroughly identified with the institution of slavery"

I entirely agree with you, such statements can be found in every one of the secession declarations. Unfortunately, those documents don't make their way into discussions involving the history of slavery and the Civil War on the right.

I think when we use the word "debate" like this, we're doing a huge favor we don't owe to a group of people who are — in bad faith — not acknowledging a set of facts. Just because a group of people hold some belief doesn't make it valid. There's no "debate" over whether vaccines cause autism, there's no "debate" whether climate change is real or affected by humans, and there's no "debate" over whether slavery was a central issue in the Civil War.

Well, apart from all those people who protested the removal of Confederate statues.

Those people will also be viewed as a black mark on American history

These delusions that hateful ideas don't have any traction in America are exactly why Trumpism has been able to succeed. There are sizable groups of people in this country with terrifying beliefs. You've got to take off the blinders.

"The Times found that nearly 20% of Trump supporters did not approve of freeing the slaves, according to a January YouGov/Economist poll"

http://time.com/4236640/donald-trump-racist-supporters/