Sure, those are all possibilities. But there are other possibilities also. For example, it could be that the selection pressure was really only high fairly recently. Most people who get schzophrenia get it sometime between around 15 to 35 years of age, and for a large fraction the symptoms come and go. So in many classical societies they would have had time to reproduce. Moreover, in some societies people with symptoms became things like shamans. So the selection pressure would likely have been not nearly as negative in the past, possibly to the point where neutral drift could account for a decent fraction of the alleles.

All of that said, I agree that it is likely that some of the alleles which produce a likelyhood of schizophrenia at some point had positive selection pressures on them for other reasons. But humans are so far from our ancestral environment that alleles which once had positive selection pressure don't necessarily have much or any today.