Who cares about "whom" any more? It doesn't contribute any useful information to any sentence.
I would say "either" rather than "both". After all, you have to choose.
It is the subject, so 'whom' is probably correct. 'Who' is fine, though.
What Trilby said.
I suppose he's technically wrong: using whom implies you care too much about sounding high class, and that can be useful information about the character of the person I'm speaking with.
the answer is 'whoever'
The correct answer is "whomever" as it is the object of the verb. In everyday informal speaking "whoever" is accepted