Is it true that a Reptilian imposter does not reincarnate into a copy of a human, but that a physical Reptilian shape-shifts into the human configuration, based on the person’s DNA, and becomes a copy of the human, and then, the human’s memory is downloaded to them?

Yes, this is a correct description of the process and the sequence of events. This is a complex melding of the prior information coming from the living human but commandeered and replicated to serve as the inner repository of knowledge and inherent characteristics, so that the mimicry will be realistic and virtually undetectable except by the closest scrutiny with foreknowledge and awareness of possible subterfuge going on.