Is the TTS voice's intonation of questions correct and you really don't raise your voice at the end? I'm not sure because few TTS engines ever pronounce questions correctly.
I tried using one way of showing the second person plural in English ("Do you all drink coffee?") But Duo did not accept it. I've tried "you guys" as well, but not "ya'll." I just don't use that one IRL at all. How do you all indicate the 2ndPP?
You don't need to indicate it, "you" means both singular and plural.
'do you drink coffee': is this referring to whether someone is drinking it right now, or can the question be referring to whether coffee is something that you drink generally (ever)?