I believe that the spider Jim we saw WAS dittos. In fact, not only do I believe that they were Wally’s dittos, but they were also helping them “through the veil” thing. Wally has been through this sort of thing before but not Kirbee and Chic. They might have needed a crash course in order to help them see through whatever illusion that has trapped them here.
Of course the question still remains, who is responsible? I’m thinking that they are lab rats in an elaborate topsider experiment. The topsider we saw before would make sense then.