It's just odd why some respawned with full gear and leveled to where I was at and others remained dead. Necromancy is all over the place right now and I just hope that one of the future patches addresses it. It's a shame that they put a new feature into a game that just doesn't work the way it was intended to work...at least I presume that the didn't intend for necromancy to be as inconsistent as it is now.
It would seem that a few perfectly safe characters to use as Dead Thralls are Orchendor, Kornalus, Ulfric, Galmar, Sild the Warlock, and Silus (though he's terrible).
Yeah, this is definitely going to need some investigation, as just in the short list of candidates you provide here I can see multiple conflicting timers and potential respawn overrides, so researching the whole lot is going to be a pain unless we can figure out a standard against which to compare potential candidates.
The system could definitely do with a patch to iron out the timer conflicts, as that's something only Beth can address; with one in place it would become vastly easier to sort out who is/not eligible as the random disappearances would stop.