Regarding the mystery of who Mehy was, Touregypt.net's Who's Who entry for him reads: "Mehy served during the reign of Seti I. What his position was is still a mystery. Many of his reliefs and images were damaged by Ramesses II, who was Seti's successor. Mehy was obviously a warrior and was normally shown in princely attire and was on many war reliefs. Even some of the romantic songs of the era talk of Mehy as a romantic hero. Mehy was a commoner according to some records and that he was a favorite of Seti. Either way, he was hated by Ramesses II."

I think the file linked from the egyptology.com webpage you first mentioned gives a good overview of what is known and what is hypothesised: Ramses and Rebellion: Showdown of False and True Horus (PDF format)

Don't you just love a good identity mystery! :)