Superluminality isn't required by all interpretations of QM. MW doesn't need it, for example. I'm not aware of a mechanism by which causality would arise from

Superluminality isn't required by all interpretations of QM. MW doesn't need it, for example. I'm not aware of a mechanism by which causality would arise from known QM, so I wouldn't expect a priori planck scale to play into it. There would also need to be an energy term for h to make sense as the relevant constant. The most compelling source of temporal direction I'm familiar with is that it's a regional cosmological entropy gradient rather than a fundamental aspect of physics, although it strikes me as suspicious that the time direction lies inside the 4-cone generated by the odd-dimension-out in our spacetime metric. That certainly lends some credence to theories which would predict the direction of time as fundamental to physics.