A 142: The Merficul weapon property does indeed make the returning damage from the Vicious property non-lethal as well.
I don't believe there's anything saying that energy damage can't be non-lethal, but it doesn't make much sense.

Q 143: When taking an attack of opportunity against a foe standing up, if one has Knock-Down (Deities & Demigods) & Improved Trip, would they be able to make an attack, trip, and make another attack (assuming they deal 10 damage and succeed on their trip attempt)?