The problem is the rotor shaft on the X12 is shorter than normal. Seems like an oversight on LRP's part. You can see in the following picture that the bearing housing doesn't stick out as far as normal motors.
In contrast, some motors don't even have an external bearing housing. But instead they have an extended shaft to make up for it...
Your best bet would be to flip the pinion around screw it down with the motor slightly pulled away from the motor mount. You may be able to slide your 1.5mm hex down between the motor mount and motor to tighten the pinion. Then slide the motor into place to check to see if you have full mesh. It may take a couple tries, but would definitely be better than 2/3 mesh.
If that doesn't work, then your stuck with 2/3...