I think the right way of understanding this problem is somewhat similar to the French usage of "tonic pronouns" after "c'est" (it is) or after prepositions. Some of the French tonic pronoun forms are different from the normal subject form (moi/je) but not all (elle/elle). When we say "it's me" or "it's her" in English what we're doing is using the "tonic" form. At least that's how I explain to myself. No need to think that the impossibly pedantic "it is I" is the correct form.