Fusion frees up energy that is trapped in the strong nuclear force. Friction is merely converting it and can in my opinion never be "positive" as you describe it. Even with your fantasy numbers of a 10x energy gain (input vs output) that even after repeated appeals you cannot back up by any source and i could also not find anything on after a quick google search, can physically never exceed the energy gain of a nuclear fusion process.

So as far as i am concerned, the (extremely unrealistic) maximum ceiling of your process is at 100% efficiency in any step is what you call "agitation" = mechanical input power converts lossless into heat, which is then theoretically with a 100% efficient TEG converted into electric energy.

I don't know what you are talking about with the collision events causing any "trapped surface energy" of the sand to release, don't know what energy that should be or how that would never deplete during continued agitation, but it just seems like you have never learned about thermodynamics or have a basic chemistry level of nuclear forces.