I'm not understanding these guesses. Sansa? Tyrion? Tywin? Cersci? Those are terrible guesses IMO!
Sansa: too nice, innocent, upset and doesn't plan very far ahead.
Tyrion: yeah sure Jeoff embarrassed him a few times but we all know he is super clever. He would gain nothing and he woul know he would be suspect number one. Too smart IMO
Tywin: only one with a possibility to do the deed but I just don't see it. He really didn't care how Jeoff ran the kingdom as long as he ran the gold and military. Besides Margery would have been the brains for running the kingdom while Jeof played king.
His own mom: least likely! He was her son and she cares for nothing in the world other than her kids. Yeah sure she likes her brother and she loves power but her kids seem to take precedence.
"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge" -Stephen Hawking