Email account terminology

Solution 1:

Username. A unique identifier of a user account.

Username isn't an attribute of a user account. Unique fields are SAMAccountName, User Principal Name, Name, and Distinguished Name (among others).

The name of a domain can be anything when used internally

It shouldn't be anything. I've blogged about this fairly extensively: https://web.archive.org/web/20200810122929/http://www.mdmarra.com/2012/11/why-you-shouldnt-use-local-in-your.html

User Principal Name [....] which explicitly targets a specific domain

Not sure what you mean by "target" but you can define arbitrary UPN suffixes, they aren't required to match up to a DNS domain that you own/control (though they probably should).

Now as for your actual questions: I have no idea what you're asking. Please re-read your post and edit it to be a little more clear, so that I can come back and edit this into something useful to you :)