Trial by combat is a fight to the death by the way. I don't think Jamie is good enough to fight a decent opponent left handed. Bronn has been making childsplay of Jaime's swordsmanship.
Bronn also probably won't want to fight for Tyrion, if he wins he'll probably be killed by a Cersei attack squad as revenge. I'm pretty sure the answer has already been given to us in the trailers leading up to this season though.
Tyrion and anyone who represents him would have to flee King's Landing upon victory anyway. As you say, Cersei will still seek revenge. TBH I think Tyrion is aware that he is done for whatever happens. Cersei would have him killed at The Wall or on the way up there. Trial by Combat is his (slim) chance at getting out of the situation alive.