The lineage Intef III = father of Mentuhotep II and Iah = mother of Mentuhotep II is more than just a flimsy theory: Iah bare the titles of King's mother and King's daughter, so she must have been the mother of Mentuhotep II and the daughter of Intef II. The stele of Tjetjy clearly identifies Intef III as the son of Intef II (see article on Intef II) so Neferu was indeed Intef III's sister (at least half-sister through her father). So Mentuhotep II was of royal lineage at least through his mother. Now since it is well known and attested that Iah was Intef III's wife, we have a more than strong evidence that Intef III was in all likeliness Mentuhotep II's father.