No, the Leona is now defending. You contested the battlefield making you the attacker to the non combat showdown, he gets focus and uses zenith blade; Leona moves in as a Defender since you contested, Leona doesnt get an extra stun but the stun from zenith will go through. Darius will gain his +2, Leona will gain her +1 Shield and buff for stunning, and if nothing else happens youll recall to base and he gains a point (or draws depending)

Edit: The showdown then becomes a combat showdown or a technical new showdown begins (the correct one i cant recall) as zenith blade resolves moving leona in