Will a hypnotist’s safety request be enough to prevent imposters stepping in pretending to be the client’s higher self?

This will not likely happen, and the reasons are the same as for all such requests. It will be paid lip service in most cases. Few, until they have had the direct experience of dealing with imposter energies, will have the intensity of intention and focus, plus the requisite 100% belief quotient in the divine, to ensure safety to pull this off without having something go wrong at some point. If the latter happens, going back to a pristine state of safety will be much harder, and the reason here is that the utilization of hypnosis is not allowing something to come through unconstrained because it is always going through the client’s mind. So the client under hypnosis will hear the instructions, will hear the entreaty to have the higher self begin to speak, and wanting this to happen, it is all too tempting for the subconscious to step in and offer some commentary. Regardless of who answers, if an imposter wishes to join the fun, this will happen and then information will be impulsed to that client to answer the clearest and most powerful thoughts that come. Imposters in the form of extraterrestrial psychics are very adept at interposing their own thoughts into human beings, to have them think, speak, and act however they wish it to be. So this is child's play for them. If the client has a high level of belief, as well as the practitioner, in the divine then safeguards may be placed to prevent an intrusion but then the problem will still remain that the higher self is not within reach and so it will be a failure to communicate in the end unless the subconscious chimes in and begins to express wishful thinking without the client realizing that is happening. Either way, this is not a productive exercise.