Do you have to halved your score in Cartamen as well ?
Yup. But you can veto a first choice by your opponent in Certamen in case your opponent chooses your bad Art. But be careful. Let's say you have Deficient Ignem and your Flambeau opponent's first choice of Form is Auram, which you're not great at. If you veto it, you might get stuck with Ignem.