(1) A verb can only have one subject (which could comprise more than one element), and that's the only role in which a pronoun can be in the subjective case.
(2) A pronoun takes the form according to the role it plays in the clause.
'these are them that he cherishes so much'
'these are them whom you are called to destroy'?
'they whom others did evil to are striking back'?
'I am him' (You can't have "I" and "he" both being the subject. Only one can be. In this case, it's "I". If "he" were the subject, the verb would be "is". "He is me" is correct. "He is I" is wrong for the same reason as "I am he".