What a brilliant analysis of this, the wasted energy of chasing Earth-bound fusion power.

I think you might have missed an ironic solution to the fundamental limitation, though you maybe touched on it, by mentioning the nature of, and the requirement for plasma.

If I interpret correctly, the pure energy of fusion is being used to mimic the gravitational effect in the sun.

Of course the energy that can be produced from the fusion process can never outweigh the energy put in, to maintain the plasma.

So, the question becomes where can the energy be taken from, to maintain the plasma field, without burning yet more of the Earth’s stored energy than is replaced by the fusion plant output?

Why, the sun, of course, we could power the fusion plant from the energy of the sun… problem solved!