Actors are people who exists outside of their work with their own hobbies and interests. Paul Rudd is getting paid to dress up and play Ant-Man only at his job, so he can dress up and play whomever he wants on his own time. (especially Weird Al)
Besides, if he actually dressed up as Ant-Man, everyone is just going to think he is a Paul Rudd impersonator anyways.
You're using whom incorrectly. The first usage should be who. "Whom" is used when there is no associated verb. The second time, whomever, is correct because there is no verb that pairs with the word. "Whos" are subjects, and "whoms" are not.
People really need to stop this celebrity obsession. He's a dad trick or treating with his daughter a few years ago. It is beyond ridiculous that people not only tolerate this, but cheer it on.
All I could think of was wow, this poor little girl can't even have a normal childhood experience trick or treating with her dad because some fuck head paparazzo is chasing them around snapping pictures of them. That's so sleazy and shitty. She didn't ask for this. She just wants to be with her dad.
Only if he goes around saying "Dubayu-EEEEENNnnnn-bee-see" while someone dressed up as Paul Giamatti constantly correcting him "no no, is Dubayu-EEeennnn-bee-see"