A pro-life freeper has told me that undesirable genetic traits do not show up with only one incestuous act but occur with inbreeding over a period of time.
I also remember learning, way back, that the ban against brother-sister incest was due to the fact that the attraction, combining siblinghood with sexual partnership, would be too strong between the two and would prevent them from ever finding an appropriate mate. This obviously refers to temporary brother-sister incest. It is clothed in authority in my memory, so it may have been the nuns.
He would be correct that inbreeding does not happen just because of a single act. It comes from concentrating the gene pool until there is no variation left to draw from.
I don’t know about your second point, but I think it was done to ensure viability of the human race and it was done when it was appropriate to do.