"This is true, but it satisfies the conditions that you claim make the Zygote a Human Being (alive, human DNA differing from either parent)."
No it doesn't. The heart will still possess the DNA of the organism which produced it.
"Under the definitions of science even a single cell can be regarded as an 'organism' of the human species."
Correct. If that organism is a living member of the human species, it is undeniably a human being, presuming your medium is scientific, logical fact, rather than moronic, blind, personal opinion.