I could believe Elijah called or messaged her, but the letter seems to indicate Kat was around (they aren't mutually exclusive exactly, but the bigger surprise is her being in town at the same time as Klaus). Also, the closing of 'love and hate, Katerina' could inspire meta all on its own.

I can just about buy Matt still having the clout to get prom king, and since Bonnie was kinda sorta there with him maybe people voted them as a pair? But if any of the girls could have been believable winning it would have been Caroline, and even that's a stretch. Like you said, they never go to school.