My guess is that the pope felt the girl's too hard to control, so he was going to 'use' her by rigging that mission to be very likely to fail.
i.e. The fake-knight was planted beforehand to either backstab them during the fight, or clean up.
If it went well, the pope could've used how the 'evil mages' killed the holy maiden as a cry for persecution against mages.
But then Zagan ruined that
So now the pope's like, fine, let's have this girl go fight that guy, SURELY this time she'll die then my plan can go back on track.