If modern scholars found evidence to this practice or not is irrelevant to the halachic discussion.

How is it irrelevant? It depends on certain claims. These claims are unsubstantiated. How is the ruling justified then? Unless you’re one of those who believe that everything the chazal said must have been true by def, including lice coming from non-living matter.

The ruling with regards to Gentiles (B’nei Noach) is somewhere in his “Laws of Kings” if I am not mistaken.

Well, find it.

If it is forbidden, it is intrinsically immoral.

Non sequitur. Care to substantiate?