OK, I was already afraid the E-Maxx mount would cause too much flex...
So the Kyosho diff won't fit the Mugen diffholder? I bought a second hand Mugen MBX5 and I still have a Kyosho diff lying around. As soon as the Mugen arrives (I don't have it yet), I will try to somehow fit the Kyosho diff. If it's too much work I will look for a solution you suggested.