You're right. Literally it means, "What are you doing?" Colloquially, however, it is a much more common way of asking how someone is doing than the literal, Πως εισαι; Sort of like how "What's up?" could be an inquiry into someone's emotional state in English. The short answer is, Τι does not translate to how at all, but the sentiment overall is most like the english phrase, how are you?
The only difference is that the first is singular 'you' and the second plural. Check a conjugation table for the verb κάνω or the Tips and notes for the verb δίνω under Present 1 (Tips and notes are available only on the full/desktop website version, they're really good, make sure to read them!).
Yes, that's the grammatically correct way. Nevertheless, Duolingo does not mark wrong sentences lacking commas. I think that this cannot be confusing, though, since the sentence can only mean one thing. Even if a comma was added (and I agree on that), the displayed sentence will remain the same, since it can't be edited, so...