He's Altmer, more than likely. http://www.imperial-library.info/pge3/summerset.shtml is a place in Summerset.
Firsthold - True. Was he Lord of it or a resident?
Speculation but if you want Occato as an evil conniver you would have to ask where he came into the picture:
In the Refugees Ifor example if the 'Master' Haymon's scout refers to was not the Bosmer Lord he served originally but rather a Master of other powers - such as magical that might have been Occato running around mastermining the rise and fall of Haymon, then of Haymon's son Mankar and finally finding himself in the driver's seat having ensured that Dagon would not wipe the entire mundus from existance (this time anyhows) ...
Not sure I see Occato as that powerful though - but who knows?