It seems like what you've done there is create two mutations (signup and delete_account) within a single namespace. You can certainly do that with the Mutations gem.
What I think you're getting at is: can you just replace controllers with mutations? That's an interesting concept that I've thought a bit about. In practice, we have found controllers to still be useful in collecting some information and doing 'wiring'. They're also nice because your SOA shouldn't really be concerned with the presentation of the response.
Finally, where do you put your mailings? You're right that controllers > models, but I'd also say that mutations > controllers > models.
What I think you're getting at is: can you just replace controllers with mutations? That's an interesting concept that I've thought a bit about. In practice, we have found controllers to still be useful in collecting some information and doing 'wiring'. They're also nice because your SOA shouldn't really be concerned with the presentation of the response.
Finally, where do you put your mailings? You're right that controllers > models, but I'd also say that mutations > controllers > models.