Convert domain account to local account
Sierra must have changed something because this did not work. The account attempts to log in, then there were a ton of messages asking for the admin password because the Library needed to be repaired. Then there was an error that a valid keychain could not be found (because the account did not have access to any of its own folders.) In the past, an ACL reset would fix that, but that's not possible with Sierra (or at least I haven't found a reliable way to do it.)

What I did instead was:


From an admin account, delete the Mobile User account, but do not delete the home folder.
From Terminal, rename the user's home folder to remove the "(Deleted)" tag. You can also rename it to anything you want at this point, such as "johnspartan" instead of "spartanj117"
Create a new local account, using the same name as the account you just edited. Select "Use Existing Folder." (If you don't see the prompt, you mistyped something. Cancel and double-check the previous step.)
Log out of the admin account, and log into the newly created account. All should be there, minus the Mobile account tag.