What FTL scenario would break causality? I can understand that FTL would mess up measurements of time/distance for that FoR. You mean "move backward" in propertime, not coordinate time right?.
So then what does negative propertime mean? Seems meaningless.
So I think that means FTL breaks the "all physics the same in all FoR", not causality.
I don't know for sure but isn't a postulate of SR that c is the maximum whatever? Seems like this would have to be assumed for the rest of the theory to "work". Said differently SR doesn't (directly) address FTL. Is that right?
Picturing the light cone in 3D (as a sphere) was pretty enlighting, including seeing that speed of cause isn't as "important" as it preceeds effect.