Not too sure for the first 2 questions, pretty sure changing from kb/m to controller and vice-versa is saved to your computer like hotbars are but I could be wrong. Can't answer the 2nd question, I'm assuming you would keep it but I haven't actually heard about that issue before.

Regarding the mail, it'll give you 20 then once you've taken everything from them there should be a button towards the top of the mailbox UI you can click to request further deliveries and it should send the rest to you within about 5 min from my experience.

Hopefully someone can give more definitive answers to your first 2 questions.