Alright, so the issue of the Create Undead ritual is largely due to the question about where the body actually came from, and if you have rights to it's possession. Fair, but the Animate Dead spell summons an undead creature for a minute, upon which time the spell is completed and the summoned undead returns to wherever it was conjured from. So, while, unnerving, perhaps, questionable, it's legality isn't as much of an issue, I am assuming?
Secondly, looking at a Summoner's Eidolon options, the Undead Eidolon is underwhelming at best, is there any chance you would be ok with me using the, much more, functional, Devotion Phantom, and call it a Karrnathi zombie of my brother, devoted to, and travelling with me to provide protection and service in his undead state? Functionally being the same as the phantom, except just changing the narrative description to a corporeal undead.