I'd say the idea of it being "too late" means it can't have been for Khan. Nothing would be too late about firing at the Reliant
, surely? Except for the hope of stopping Khan from firing. But that would never have been Kirk's goal, because he ordered the phasers only after
Khan had fired the torpedo...
It would still be a desperate last grasp at a particularly thin straw, only made worth the try when Kirk realized shields were not going to get up any time soon.
In the nebula, there would be no shields. But there would be no way to target anything much, either - Khan even missed a starship-sized target at twenty paces! The best defense against incoming torps in that soup would be to stay vewy vewy quiet and hope the enemy finds nothing to aim at.