They couldn't use the MDS as proof anymore since the anima had already hidden themselves in the larger group of anima. In order to pick out the two imposter anima from the large group, they would have to somehow make them unleash their actual mana capacity and that would only happen if they started a full-blown fight.
And if Mark tried to strong-arm the Russians, then the Japanese government might even think that Mark just used his power to force the Russians into talking! If that happens, then Mark was definitely going to be thrown out of the tournament and the two anima that had snuck into the anima horde would be able to move freely without Mark there to stop them.
