Thanks for the thoughtful article. I conducted a less rigorous experiment but similar experiment: I joined a local community FB group that wasn't specifically focused on politics but included lots of folks on the right. I was always polite and folks were polite in return. My central question to the group members was, "Can we find common ground?" I found that most of them just weren't interested in carrying on conversations with me. To the extent that I did get any response, it was along the lines of, "Perhaps, but you guys must first admit that you're wrong and apologize for how badly you've treated our President for the past four years." I followed up with some areas where I thought there was potential for finding common ground (rural internet, rural education and health care, more and better jobs training) and was kicked out of the group for my troubles. I don't lose much sleep over my expulsion.
One quibble I'd have with your article is the omission of race as a factor. You do say that the white supremacists are the most frightening group on the other side but I think race is central to ALL the energy on the right. When you say that you doubt there's much we can do to change the outlooks of 90% of them, I agree. I'd recommend a book, "Dying of Whiteness"; it essentially says that many white Americans are willing to commit suicide rather than see POC obtain any fair or just treatment at all.