Non sequitur: The Church may very well have received magisterial authority from Christ; but even if Christ "cannot err" (which btw has to be demonstrated; you seem to take it as axiomatic), it does not follow that the Church cannot err -- unless you wish to assert that with the authority to teach comes all the content of any future teaching, a bizarre shift in the meaning of "authority."

As for slavery, etc., whatever you wish to say about "Christ Himself," the historical Jesus was an apocalyptic prophet, who expected the eschaton after only a brief wait, and so was not interested in reforming society, as though society was expected to persist for a long time. There were all kinds of subjects, ethical and other, about which he had little or no interest. So to assign to "Christ Himself" direct authority for teachings and judgments that Christians later devised for themselves, however fine, is fanciful and fictitious: defensible perhaps, but hardly to be imposed as a criterion.