How did Thorn pull it off, though? How did he know there was going to be a ball, that Larry was going to be there, get close enough to cast Imperius, etc.? It was more believable that he knew what was going on at Charmbridge when Max was there. Of course, any of the teachers could be part of the Thorn circle (Dean Grimm, maybe?). I do agree that using Larry was probably not a random choice, because Max could have told their father about Larry pretending to use Crucio on Alex.
On another note, I'm embarrassed to say how I reacted when Larry asked Alex to dance. Let's just say there was high-pitched screaming involved. The 'ship teases in this chapter were horrible...just horrible! :) So, getting over that, it was disgusting that Thorn basically programmed Larry to deliver a message. It was good to see that Alex at least was a little concerned about the real Larry if her father was using Polyjuice. Not just because I think these two will eventually become at least friends but mostly because it shows that Alex, though single-minded and desperate, is not ruthless in achieving her goals like her father seems to be.