For a pointless academic question you elaborated exetensively. ;)
Last I looked there were cities around the desert. Xykon would get more people. And he has Redcloak for organising the Not-dieing-until-I-allow-it-thing.Most living slaves would be dead (of thirst, or sunstroke) within a couple of days on the job.
Zombies are bad searchers. But we are not talking about zombies anyway.There's a limit to how many zombies Xykon can control, and I'm not sure how effective they'd be as searchers anyway.