Friend of mine had an interesting idea today about Cersei's trial by combat. What if Tommen becomes the champion for the faith? Cersei obviously isn't going to let Tommen get chopped in half by the Mountain, so she would pretty much be forced to confess. High Sparrow wins again.
Or, the other option, HS declares that the trial by combat can't be stopped at this point, convinces Tommen the gods will protect him(because Tommen is stupid), and when Tommen dies with no clear heir, HS takes over for himself.
So either way, High Sparrow wins.
I think you're right. Cercei has mentioned trial by combat twice. It has to be important.