That same book by the way mentioned the unlikelyhood of men and mer being able to mate with Orcs, but Orcs are just changed Mer anyway.
http://www.imperial-library.info/content/racial-phylogeny
I really don’t get the need for definitive proof in a game where lore purposefully has tons of holes so that it could be changed. The fact that there has been no recorded
examples of it for thousands of years somewhat puts it out of the question until new lore is introduced. Considering that some species of Khajiit are just like Bosmer when you shave them I’am sure that cross breading was tried by some sick Telvanni.
edit: Orcs can reproduce with human-ish individuals, the Pale Prince is an example. The mother was an Orc the father…wasn’t. It is however unclear if an Orc male can impregnate a non Orc.