Who was shooting at Carleton?
Sir Guy Carleton headed south from Quebec attempting to support the loyalists, and ran into rebel cannon fire. The rebels opened fire before he was really in range, and so he was able to withdraw with only one fatality, and decided to return to Quebec. Thus, Arnold failed to take Quebec later. While it's usually assumed they were some of the green mountain boys, it has never been explained why some unnamed and unaccounted-for cannon-toting rebels happened to be where they were.
Not really a major point, but you asked for unexplained things.