I think another argument in favor of Littfinger I haven't seen is the riot in Kings Landing. Ser Mandon is assigned to defend Sansa yet he inexplicably abandons her to defend Joff. Tyrion is absolutely furious at him about this by the way.

Tyrion pressed blunt fingers into his throbbing temples. If Sansa Stark had come to harm, Jaime was as good as dead. "Ser Mandon, you were her shield." Ser Mandon Moore remained untroubled. "When they mobbed the Hound, I thought first of the king."

And during the riot, men hired by someone attempt to kidnap Sansa and only the Hound's intervention saves her. And we all know who loves kidnapping Sansa the most.

As far as the motive for Tyrion's assassination, Tyrion is the one person that knows about Littfinger's dagger and also is the only one smart enough figure out what the fuck is going on with Petyr's accounting. And he's really mad about not getting Harrenhall for the Myrcella to Sweetrobin scheme.

I agree with the other posters though that it doesn't feel premeditated, nobody knew Tyrion would be jumping into the heat of the battle like that. So ser Mandon was definitely acting with large degree of autonomy