Were would be more likely than are in this situation, I less it happens quite a lot
Disagree, as we have no context. If it were in the past usually sebelum or dulu is added so we know that we are talking about the past.
"They asked to sing at home" was marked wrong. How then would I word it differently to say this?
The course is trying to teach the importance of the prefix di- with this sentence. I think that's why it marked you wrong.