Is cosmopolitan, interracial future the best vision for global justice?

Yes. With the caveat "as long as it's under MY terms." Ask someone else. The answer and caveat will be the same. So how can we keep the "Yes" and remove the caveat?

On the other hand - is that possible? Is it human nature to be tribal? Is there a limit to the size of a tribe? Or is it possible to form larger and larger "tribes" until there is only one?

Individuals are raised (educated / indoctrinated) to belong to a tribe. But if taken at birth and raised in a different tribe their tribal allegiance would be different. But being within a tribe doesn't mean that there is no conflict within the tribe, however small. (Dysfunctional families, for example.)

"Tribes" can be formed from disparate sources when a common enemy appears.

So, unless there is an extra-terrestrial threat to the Earth that causes nations to face the common enemy I see no way to achieve a cosmopolitan, interracial future, at least for several generations.

The push to create a United States of Europe has all the trappings of a grand design (built to the terms of the European Commission members). The problem is that the EC want a U.S.E. in five minutes. Five generations maybe. And then could the USA and USE merge? Another 5 generations. And maybe the rest of the world, slowly, slowly?

But is it possible for global justice to exist without a cosmopolitan, interracial society? Well, then we get onto the definition of "justice". My justice is that it's correct to amputate someone's hand for theft. His justice is that the thief needs counselling. Your justice is that it depends on the enormity of the crime.

So it seems that the ideal of global justice is a chimera, and we must muddle along as best we can.