dont use keystone on a layer- dont even touch it-
dont mess with the keystone visual fx layer parameters -
use keystone on sub-mix - then all perspectives and scales on layers stay aligned - and you can playback multiple layers on the same object - keeping everything aligned - just by routing a layer to an output mix - its much much easier.
use one sub-mix for each object on stage-
you will need an alignment image which allows for easy adjustment for every object on stage - probably matching the aspect ratio of the objects on stage.