It's there, after the parameter list.

If you're connecting to the instance "SERVER\Instance", then the login error will be in that instance's error log or, if you started SQL from the command line, will be on...

-S <server> -E should work. What's the error from the SQL Server error log?

Did you tell it which server/instance to connect to this time?
Just like any connection, you need to specify the server, the authentication (trusted or username and password) and...

sqlcmd /?
and  look at the options it presents you.

A restore recreates the DB as it was at the time of backup. You can't split/merge or change files during a restore.

-U on sqlcmd is used along with -P for SQL logins, not windows. Use the parameter for trusted connection and make sure that you're logged into Windows as a local...

Have a look at the error log, see what the detailed description is for the login failure is.

Also make sure that you don't have something like SQL Agent or other services attempting to log in and taking the only login available.

Once you connect via SQLCMD, you can do anything. You're a sysadmin. So the same T-SQL you'd use normally to create a login and make it sysadmin.

Unless you have some custom auditing, SQL doesn't track when someone last logged on. If you need that, you'll have to set something up. An extended event session will probably...

When the instance is in single user mode, the only accounts that can log on are SQL sysadmins or windows users who are members of the local administrators group. If...

If you specified accounts at install, they'll be sysadmin. Log in as one of those and add extra accounts or change the sa password.