The key to this problem is to see that point at which the sphere fails to complete the loop is not when it's speed is zero ... but when mg = m omega squared r , omega here is the rotational speed around the loop ....r is not the r in the question

We have another omega which is the rotation of the sphere itself about it's center of mass ..

At the point of failure there is inertial energy stored in the **spin** of the sphere , as well as energy stored by virtue of it's **velocity**

It's an energy balance problem ...( height of mass of sphere at failure is 2R-r)