I am a bit confused about the shared computer question. Separate user accounts are a security feature that keeps malware from so easily moving from one account to another or to root/administrator control. It is not just a way to keep your porno collection from your kids. I understand why multiple accounts would be of less value on a private device like a smartphone, but what would be the point of a single user account on a shared computer? Windows has been offering fast account switching, leaving other signed on accounts active but not accessible for some time. Are you saying you prefer the convenience of using only a single account for everything on a shared computer? That is easily done by just setting up one account and using automatic login, thus letting anyone who can get to the machine get to the account, without even knowing the password--the ultimate in kamikaze security carelessness.
As for shared email accounts, that can be set up in the email reader if you use one that is not locked in to the computer user account. Thunderbird can be set up to read dozens of email accounts from the same inbox if you want everyone to have access to everyone's email, and can be set up to run on multiple accounts or machines if that is what you want.