I have a sneaking feeling Yuuichi was the one who wrote the letter and is reading it out himself. It's such a him thing to do: the intense psychological warfare.
Don't get me wrong, I don't think he has the key, but rather he's making people play into his hands.
I was thinking the exact same thing lol, I even made a comment about it just now, without having seen yours (￣∇￣")
"We don't have anything to lose, so we just bluff and see if we can trick him"? Probably :D
That theory makes sense considering that Yuuichi has a way turning everyone on their heads by stirring the pot.
Hmmm the game is getting interesting, I can't wait to see the look on Kei and Kuroki's face when they are beated by Yuuichi yet again.