Cersei surely would not let The Mountain kill him. He's the only person she wouldn't allow The Mountain to kill in this situation (sorry Jaime). She might even... sacrifice herself to save him.

And Tommen's so dumb that he might actually think he could win the battle because he's got righteousness on his side. Margaery might even push for it knowing that it's a certain way to defeat Cersei and consolidate her own power.