"Please go home quickly."
I think it's because they haven't introduced the other word for "return" yet, 返る.
I thought the same...I think it shouldn't be marked as wrong answer.
Just to be sure, I tried putting the adverb after the verb and it was marked as incorrect; since I know the Japanese word order is rather flexible, I would like a confirmation/infirmation for this one.
I suspect it should indeed be placed before because it is specifying the action, but you never know.
I might be wrong, but I don't think that you can put an adjective in front of ください. As far as can remember it was only verbs. If you want to use that word order you just have to add して before ください.