"Five dollars and eighty cents" should be accepted here. The context could be understood to be 美元 (or even 加币, for that matter — the name of the currency doesn't even have to have 元 in it).
(Also, there's no reason to capitalize "yuan", just as there wouldn't be to capitalize "rupee" or "peso" or any number of other currencies.)
I think it's possible that since I wrote my comment, Duo has added my suggestion to the accepted responses. It's been a while since anyone said it wasn't accepted, and the answer lists have expanded a lot since the course came out.
To my mind your suggestion for the Chinese wouldn't really make sense, because the US isn't the only jurisdiction that uses dollars as a currency, though it might make sense if the English referred specifically to American dollars.
Yes, in the same way that 元 is, for 钱.
This is another question in which a fractional monetary number should have an answer written with two decimal places: "5.80 Yuan". Otherwise, the user gets corrected for having miswritten the answer. Since an answer with "5.80" is accepted, there's no direct way to report this issue.
I think when one is genuinely sharing one should not have to apologise for commenting on a post. Sometimes when going through the lessons I feel that there are inconsistencies and when I discus these with my wife who is chinese she at times cannot explain my queries and that is from a person that studied languages at Beijing University.