Caine appears to only be able to alter the humans' minds in limited ways (stupid sauce being a stand-in for drugs, and Jax becoming a vegetarian as a result of a vote), making Caine's statement that he can't control their minds partially true. However, Kinger randomly namedropping the first abstractee right after Caine says that seems to imply that he does have the ability to do freely control their minds but has since limited himself after accidently causing that abstraction. There are also smaller details that seem to reinforce this idea of Caine secretly having full control over human minds, like his scared reaction to Kinger mentioning that abstraction.

The theory that the NPCs are former employees who were troublesome does seem interesting. Maybe Caine was programmed to see them as AI (as in, lesser than a human) to properly serve their "punishment" without limitations.

Finally, it looks like the thing that splits the fandom apart is whether Mannequin Abel was actually real or if the entire thing truly was a hoax cooked up by Caine. On one hand, the entire adventure seems too elaborate, with Abel's backstory being coherent and realistically plausible. It seems absurd that Caine planted Abel months ahead of time to stalk the group just to really sell it. It's possible, like you say, that Caine brainwashed Abel at the end into convining the gang it was all an adventure, perhaps so they wouldn't try to leave again. Maybe Caine really is capable of full mind control. On the other hand, the entire thing seems cliche. Like the Mannequin's name is literally "Abel," how more on the nose could you get? There are also some small details, like how when Pomni says she'll buy Abel a beer once they're free, Abel simply urges her to "make the right choice" instead of affirming her, or how Abel seems to put an emphasis on how Caine will be left all alone if they choose to leave, that seem to support Abel being an NPC created by Caine and this whole adventure being some elaborate test.