If A is impossible, then B is impossible too, but Guess what? B happened...?

Yes, inbreeding causes mutations, but not all mutations are detrimental. Some mutations can be beneficial like the fusing of chromosome two. I do not know how they teach biology at your school, but at our State UC university we are told that humans arose from 2 descendants. We just do not know which and where those descendants are located, but we are getting closer everyday to finding out.