Well, this certainly leads me to change my view, but perhaps not in the way you think.

At first I doubted this evidence (and I still wonder how much of it people could replicate). I would expect contact with different people to reduce fear of outsiders. Indeed, Putnam suggests as much and this later source confirms it -- having neighbors from a different ethnic group increases inter-group trust. Yet the same sources claim that an ethnically diverse neighborhood reduces trust in 'hoods and neighbors. I didn't get the impression that racial prejudice started out strong enough to explain this, though I could be wrong.

I think y'all may have buried the lede here. If these and the other results you quoted hold, then maybe all altruism comes from tribal instincts and ethnic diversity interferes with our evolved tribal sense (until the mental categories change, about which more in a second). This might explain the greater participation in marches and reform groups. The loss of a tribe leads to desire for a new one.