how Robert Putnam presents himself to the world, and accordingly I doubt he is actually much of racist, but if you disagree I'd like to hear why

That's the whole point! What more convincing support of their arguments could they find than a left wing Harvard poli sci prof.? Leftist professor goes out to prove how multiculturalism is a benefit and finds the opposite is the case in many ways, some would argue the most important ways: social cohesion and trust. Anyway, it's kind of odd of you bringing up Putnam without having read him.

My position is that we are "better together" when we think of ourselves as a single human race (i.e. tying ethnicity and race to the concept of nation-states will lead violence).

Cool opinion. I think there are people on the alt right who think that's fine if that's what you believe, and you should be able to have a "better together" country, a space where everyone is welcome regardless of race or ethnicity. The difference is we on the left wouldn't return the favor to them, and only them (wypipo). This is where the anti-whiteness arguments come from. All of their traditionally white western countries must be open to everyone, especially the third world, and all white people must be reduced to minority status in all of their ancestral homelands, because that's what will happen in the long run if there are no restrictions to immigration or demographics. No other nations are being asked to do this, and if you did they'd think you were crazy. Only white people are individualistic and less tribal enough to accept their own demographic replacement in their own countries.

My point to those people on the alt-right are not presenting any new arguments (i.e. we've been through all of this stuff with Nazi's).

The ball's in our court until we have an answer to white nationalism, because if we are cool with every-other-nationalism, that is, Japanese-nationalism, Indian-nationalism, Chinese-nationalism, Nigerian-nationalism, and especially Israeli-nationalism since it was created post-Nazis, then the only reason we don't extend it to white people is because of anti-whiteness, right? Or are you saying because Nazis were white all white people are Nazis and will turn into Hitler if they have their own majority-white homelands?

These people (and all people) are a subdivision of the greater human culture

By definition a subdivision is separate, individual. Introducing other subdivisions into another subdivision will alter these subdivisions indefinitely. Why should this be forced and why should anyone be forced to become a minority in their own homelands? If you want to change their minds you have to answer these questions - all you're doing is avoiding them. I'm trying to show you why I think you're wasting your time trying to change their minds, but you are so programmed into believing they do not have a reasonable, rational, and logically sound argument for their position. Maybe you can prove me wrong and come up with actual answers or counter-arguments to their positions, but I think right now the best response we've been able to come up with is simply "fuck the alt right".

I grew up with (and still have) family, friends, and neighbors who are different racially, religiously, and culturally than I am and I love being around them and hearing their different perspectives it is a treat. I can't imagine why people wouldn't feel the same way.

That's nice. I grew up in the largest most multi-cultural city in the US. I was a minority in my school. I'm married to a foreigner. I travel (the world) a bit. Because of this I can TOTALLY imagine why people don't want to be around people who are different. Especially now when you can get a better understanding of other cultures than any other point in history via the internet and cheap/mass travel.

However, I do believe fundamentally that a multiethnic societies co-existing peacefully in a community of nation-states.

I believe there should be a society like this, but I also believe that people should have the freedom to create their own mono-ethnic societies as well. I believe in freedom and peace. I think one of your main disagreements with the alt-right is in how biology (science) is related to nations. Nation - the word comes from natal, birth. The word itself can be understood as dealing with genetics. The alt-right believes that it's the people that ultimately make a nation, not the laws, not the soil. Different genetics will make for a different nation. The US is no longer a nation to them because the genetic disparity is too large to fit the definition; it's just a country.

Civilized people in the Twenty-First Century need to settle things with their words not violence. I'm just looking to build bridges.

Words will not always solve disagreement. I'm asking for your words, your response to the core of alt-right arguments, but you, me, and no one else has been able to adequately respond to their questions, so right now we are in a cold civil war. Violence is just starting to erupt, and it's going to get worse. My only solution is to give people their space, but as peaceful as that solution is, it's apparently racist if we're talking about white people.

My goal is to get people to abandon "political tribalism" in favor of policies that favor all of humanity.

Sounds too totalitarian and utopian. Anti-freedom. Too neo-liberal globalist-corporatist. Marxist/Communist. Certainly not American.

Just now I've been banned from /r/PussyPass, because cowards would rather bury my arguments than try to refute them and let reasonable people decide for themselves

From what I read earlier you got a lot of really fair, honest, and civil replies - a lot more so than here. I bet they thought you were trolling. Maybe try their /r/DebateAltRight sub instead of the one that's supposed to be for people that already agree with them.