Well I would expect Wallace shouldn't be able to use a Reality Marble at his level of Mana. If he did it shouldn't even last long at all honestly. Even the attack would probably leave him incapacitated for awhile. I'd personally just advise increasing the power of his sword instead of using a reality marble. 

Also I'm thinking of having a Magus family based in the Americas. Isn't this Grail War in North America?