Why calculate the trajectory for the sounds/slow down effects?
If you split the act of slowing down the camera and playing effects into sections upon approach to your squishy victim then you can essentially play them on condition of their proximity.
This is a great example that comes to mind. The proximity slow down effects employed in Peggle
The ball gets close to the peg and starts slowing down. This is actually based completely off proximity, no trajectory involved. If it gets close enough to the last peg it starts a drum roll and slows the game-play down, if it does not hit the peg and goes out of range you hear "Awww.. :(" and all returns to normal, but if it collides you will get the full fledged Overture and winning screen.
This is a great technique if you approve of making your player go "Holy cow, I nearly got hit!". If you have a start-up sound/effect for a death cam that will play before impact then simply trigger this when the stone is close enough that even if the player moved at maximum speed in any direction - they have a great change of getting squished.
Say your rock is falling and you want to begin the slow down effect to ~4m away from the player: You start by triggering slow-down, move your camera and start focusing on the player - if the rock is then interrupted by other debris then simply exit the slow down focus and your player continues as normal.
This is essentially how Peggle handle their slow down effect and trigger the end game music and is perfectly viable for using on your camera/rock/squishy pre-empting death-scene scenario.