What is nominal shaft diameter of the armature? I would compare the adapter's diameter to that, not the tranny's. If it is smaller it could be a potential weak point... but really, how much torque is it going to be called upon to transfer?
It's hard to say because the emotor can put out so much instantaneous torque from a start that it can easily chew things up. Metro, maybe you should put some kind of damping circuit on the accelerator potentiometer to limit sudden increases (if your controller doesn't have this already). That "soft coupler" you have is very clever.
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein