Almost all of the ionosphere's total thickness is below 550km, so the lower altitude doesn't matter. Starlink sees less impact from the ionosphere because the carrier frequencies are higher, and dispersion is proportional to 1/f^2.
I don't think that's true? Plasma density goes up with altitude out to quite some distance, though it's nonlinear. And in any case it's a refractive thing. Having a somewhat lower index doesn't make much difference when one lens is 40x thicker than another.
