Again, the Green function for the Dirac equation is causal. So I don't see how your Section 3.3 contains the argumentation.In the wave function, there is no actual motion of particles, just a motion of probability amplitude.

Moreover, everything happens in afixedframe in which the spatial Fourier transform is performed; one cannot argue with time dilation or length contraction.

From the Schrödinger equation, the support of ##\psi## is ,after sufficiently short but positive time, definitely the union of the initial support and that of ##H\psi##. But there is no reason to suppose that the fairly arbitrary ##H## allowed by the construction in K/P leads to an ##H\psi## with bounded support; note that ##M## can be quite arbitrary. To preserve the relativistic probability interpretation in concrete cases, one would have to construct very special ##M## that preserve a bounded support - but this seems quite a nontrivial mathematical task.

And what is K/P? I guess I missed something.