No, the purpose of the cone point is to force the point into the shaft. The more the set screw is tightened, the deeper the cone is inserted into the shaft, making it more and more difficult to uncouple the motion of the adapter and shaft.
If you had the ability to create an adapter that had a D-shaped cutout with the same dimensions as the D-shaft of the 25D mm motor, then that would be an improvement.
By the way, have you considered that you might not be tightening the set screws enough? If you are using the small Allen wrench we include with the mounting hubs, you might try increasing the lever-arm of the wrench to give yourself more mechanical advantage.