>I get that for many states, if the Democrats had the 55, then current gerrymandering means that they might only get 2/7, or two out of the nine candidates, which seems wildly off no matter how you look at it, but still, I have trouble identifying the principle of what fair actually is.

Optimize the variance of election outcomes. If pundits and such can't predict how the election will go, then and only then can we say voters really spoke.