Yes, that’s normally a good rule to follow, since “whom” is for a sentence’s object, but this is a special case. The clause in question is either a salutation that has no subject or object, and so either “whoever” or “whomever” is correct, or it’s a subject clause (a noun phrase, really) with an unnecessary, stylized “to” for the sake of comedic impact, in which case “whoever” would be correct.
It says “to”. To whom.
Yes, that’s normally a good rule to follow, since “whom” is for a sentence’s object, but this is a special case. The clause in question is either a salutation that has no subject or object, and so either “whoever” or “whomever” is correct, or it’s a subject clause (a noun phrase, really) with an unnecessary, stylized “to” for the sake of comedic impact, in which case “whoever” would be correct.