Electric potential

So the definitive answer to this whole issue is just a lack of clarity in wording:

Charging an object to an electric potential of 866 V
means
Charging an object to a charge Q such that the electric potential on the surface (NOT at the center nor at a point charge) is 866 V.

The problem states that it is a conducting sphere which means that it has a uniform charge density. There is no distinction between the surface charge and the charge at the center. If I understand correctly, you problem actually stems from wanting to understand too much, too fast. Your simple problem doesn't really make a distinction between the charge on the sphere and the charge in it (hence placing the particle at r >> R), because its trivial to the results of the problem and incredibly complicated to define in most cases. Effectively, this idealized sphere is a particle.

There are better definitions around I'm certain, but I would attempt to define electric potential as a measure of the potential resistive or attractive force exerted by an electric field on a charged particle.

To clarify your graph, yes but no. You're delving into the realm of super colliders trying to split particles by running them into each other at relativistic speeds. Here there be complicated pomegranate were people just really shouldn't go without a Ph.D and I won't presume to try and explain effectively. With only the Maxwell's equations at your disposal, I don't believe it's possible to accurately explain it.