"Listen well" is proper grammar. "Well" is being used as an adverb, describing HOW you listen. "Good" is an adjective, describing or telling "WHICH ONE" or "WHAT KIND," such as "The good boy gets the candy." Take the good pencil, not the bad one." You can't really have a "good listen," per se.
I believe you are right. I think it's a case of DL doesn't put every possible combination of words in the list of acceptable answers. They have to be manually entered by someone. I expect that if enough people mark it as "should be accepted", it might eventually get added.
I’m just a student also, but I suspect it’s an indirect way of saying it that fits with what I know of the aloha attitude. Saying YOU listen could be perceived as being a bit rude, so one takes the safer way and refers indirectly to the ears. Kind of like in English when the teacher says “let’s all be quiet” when they clearly intend for only the students to do so.