But then I started to think about it and it doesn’t seem exactly right either. Causality kind of implies interaction, and interaction implies mass (no?), so causality as such must necessarily be strictly slower than C. That is, the speed of the propagation of the wave function between causally connects to events is C but causality as such are the interesting things happening at those events.
Not necessarily. Photons, for example, are massless. But you're still pretty much correct. Information cannot travel faster than C.
Though there may be some special cases that I'm not too familiar with. One I'm a little curious about is entangled quantum particles, though I'd have to do a little reading to be able to say anything further.
In the past/future you find all the stuff within the corresponding light cone. The stuff outside the cones can flip between being what seems (non-Relativistically) like it should be past and future, can be either depending on your motion relative to it, so an extra tense for spacelike separation is something I can imagine a sufficiently advanced civilisation using.
If we have two alien neighbour planets aranged so, that we are in the middle between them.
Then we could see both aliens' election results at the same time, knowing that aliens still have to wait a bit before they know each other's results.