S P O I L E R S belowThrones SPOILER:
(I repeat - SPOILER SPOILER SPOILER)
Jaime does not participate in the trial by combat. Neither does Bronn. And no, neither does Tyrion. The person who steps in for him is awesome, and obvious if you stop and think about it for a bit. Who doesn't give a s*** about what the Lannisters think, is a renowned swordsman and cunning warrior, and has cause to try to mess up Tywin's plans? And is currently in King's Landing?