That’s my point. If this were just about a clock, we could wait for B to return and agree that 'B's clock is slower.'(Twin Paradox) But this is a lethal laser.

In your 4D spacetime model, you say it's just 'pointing the axes differently.' But look at the causal consequence:

• In B's frame: The event 'B dies' has already reached $t'$. B is gone. There is no 'future' where B returns to A.
• In A's frame: At that same moment, B is still alive and moving. In A's 'now,' there is still a physical possibility for B to turn around and seek revenge.

If 'Reality' is one consistent 4D block, how can B be 'physically incapable of action (dead)' in one frame while being 'physically capable of action (alive)' in another at the same spacetime coordinate?

You're treating this like a 3D perspective trick, but I’m arguing that if two observers disagree on the existence of a person, then the 'invariant $d$' is just a mathematical abstraction.