On Serenia, inside the boys face (when you pull the red lever after encountering Achenar at the broken bridge/walkway) where the life stone is supposed to be, what is the purpose of the light reflector you can move? Is it a cut puzzle? I don't understand why'd they'd go to the effort to put a moveable object in there to have no purpose.

It's possible it might have been a puzzle they cut for some unexplained reason (time, etc.). It might also be there solely for world building purposes (I think there's more bits, on Serenia in particular, that are interactive but serve no actual purpose to the player's goals).