Oh my, it sounds awkward, but I cannot explain why! :D

If I would like to use you here, it would be to stress it, and I would write it at the end...

"Quando bevi latte tu?"

or (but the meaning would change "Da quando tu bevi latte?" (Since when do you drink milk, meaning "When did you start? I never noticed it)

In general, in Italian the subject can be always implied, we use it especially when we want to state that a thing is made exactly by that person, some kind of surprise or other feelings.

It is also correct.bevi only come with tu. So no need to stress it.

Depends on the subject. The verb will conjugate according to the subject.

Bevi is informal and beve is polite and to be used with people with whom we are not familiar.

Voi is "You Plural". It is to be used when addressing multiple people at once.

Quando kind of sounds like "when do", or at least looks a little similar. It's not perfect, but whatever helps it stick, you know?