Would brainwave monitoring of a client in Delta sleep also show perturbation when a channeler is being duped by an imposter, because an imposter will also be able to impinge on the test subject’s brain waves psychically, mimicking a direct connection by the channeler not actually taking place?

This is very much the case so unfortunately your creative idea for a way to validate that you're channeling takes place is helpful up to a point in showing there is a direct interaction with the test subject but unfortunately it will not differentiate you from the many false channelers who could carry out such an experiment and pass with flying colors because their imposter will be onto the game and will see to it that whenever the channeler reaches out to interface with the test subject this will be done by the imposter minding that channeler as they can do this readily with their psychic ability and that will look much the same. There will be a brainwave change when the imposters’ consciousness shows up to engage with the mind of the test subject and then there will be arousal and a change in the brainwave pattern. This cannot be used to validate true channeling from that of an imposter duping a channeler.