The human ear (in situ) has been reliably shown to engage in backward masking.
The human psyche engages in defense mechanisms daily, routinely...there are a multitude of ways by which we "gate" our sensory input.
Yes, but the defenses could be rendered useless by using mind altering drugs like LSD, BZ nerve agent or even sodium pentothal to have a hypnotic control. Any person can create the Manchurian Candidate if they really wanted to.