As for its antecedent, it makes more sense to say that it has the witch in this play as its antecedent than the subject phrase I. I is not anaphoric to the expression the witch in this play, as the interpretation of I is independent it.
if one assumes that the subject phrase I is the antecedent of the predicative phrase the witch in (3b), then one does not get access to the desired information that the predicative phrase the witch refers to the same witch as does the prior phrase the witch in this play.