One need never use whom, and if one is even a little bit dubious about a situation, one should certainly not use whom there. That's the rule. The simple rule. If you insist on zombie rules, be aware you're late to the game, and there are lots more zombie rules out there already. Whom has kicked the bucket, shuffled off this mortal coil, run down the curtain and joined the bleedin' choir invisible. This is an ex-pronoun. Let it lie in peace. – John Lawler