"Hello, who are you?"
Translation:Salve, quis es tu?
I don't see any reason for claiming that it is better without the "tu". In literature, you encounter "quis es tu?", "quis tu es?", "tu quis es?", "quis es?", "quis tu?", "quis tu es homo?", "quis tu homo es?" The last of those is actually the most common.
When I was selecting words using the word bank, I chose quid instead of quis and it was marked as a typo, even though I had chosen incorrectly.
No, "salvete" would be used for addressing multiple people. But "tu" is "you" singular. As in there being only one of you.
Similar to not using "How are you all" instead of "How are you" when talking to just one person.