I agree with your analysis of Neanderthals. We would have to see a live one to determine whether they are truely human, but it seems to mee that even if we were presented with one, it would be very difficult to judge whether it were fully human. And yes, I mean "human" in the ontological not biological sense.
But are these reasonable grounds from the perspective of evolutionary theory? There was a recent pop-science article going around claiming natural selection and evolutionary change was very much still in effect among modern humans.
And don't certain transhumanist fantasies about growing new organs and brain lobes posit quite a few difficulties, even if they aren't ultimately feasible in practice?