Hm... I have no idea about the alibi or trick, but I think it might be the blind girl's tutor-- the tutor said that she wouldn't be able to use a bow for a while. As Kyu might have realized from seeing Ryu play, bowing is done with the right hand, which is why what he realized was not related to the piece. The bowing helped him realize what he'd heard-- that the girl wouldn't be able to use a bow. However, at that point I'm pretty sure the tutor was only told that her arm was broken-- which arm wasn't specified, but the bowing comment betrays that she knows it's the right arm (fingering is done with the left hand). Anyone have any ideas about the trick or alibi?
UPDATE: Megu says that the hallway lights had to have been turned off beforehand so that the culprit's eyes could get used to the pitch darkness before turning off the room lights. However, in chapter one you can see the culprit turn off the hallway and room lights pretty much without a delay in between (unless there was a possible not shown time skip). The only person who doesn't need time for their eyes to get adjusted to the darkness is the blind girl, who is also the tutor's only alibi. They might have been working together...
The flowers might have been placed to explain/hide the lavender scent, I still have no clue why there was a lavender scent in the first place though. Alternatively, the flowers have a trace of the smell of lavender because there were lavenders among the flowers before, which were removed. This doesn't explain why the flowers were placed at all, though.