For anyone wondering, yes, Aoba should really be saying "Yagami-san", but "Kou-san"'s what the author put down so meh. Dunno if their formalities change during vol4 or whatever.
The message was different then the original unsent one so I got the feeling she wrote and sent that one to herself
That was actually just a TL gaff on my part. Basically the Japanese version just truncated the earlier message to "Aoba-chan ga suki desu." In retrospect I should've changed that earlier bit to "I love Aoba-chan when she tries her hardest" or something to consistency but meh