Others have dealt with the rest, I'm just going to concentrate on this one bit:
Having said that, we also have done some tests, because of which we have found out that the existing population of the world are all decendants of a single human being
This is incorrect. No-one has ever performed a test for the existence of mitochondrial eve. Her existence is
assumed based on the statistical near inevitability of it being so. What they then did have assumed she existed is apply the
science of genetic molecular clocks to the data gathered on variation in mitochondiral genomes across the human population to calculate when she lived.