Very complected question.
As for getting 365 and a new email should not effect anything. It is a MS account required for the product and what is needed on Win 8.1 or 10 to be able to use the store within the OS. You don't have to use the email if you don't want to.
For his three other emails he can add all them accounts to his tablet no different then you would a cell phone. I myself have an MS account.. a gmail account... yahoo account and a private domain account on my phone.
You can forward emails from any of them accounts to any other account like all my emails can be forwarded to my MS account if I wanted.
Or he can download OfficeSuite Pro and edit his documents that way skipping 365 which is what I would do. Screw 365.