How to immediately terminate a SPID with a very long rollback time estimated?
Let that thing rollback. There's nothing you can do to stop it or speed it up. If you shut down the server or "trip" on the power cord, the rollback will just start back up again when you turn the server back on.

In any case, if you can restore as a different name, you don't need to rename the data file. First, see what files you have:

RESTORE FILELISTONLY FROM DISK = 'C:\path\file.bak';


It will indicate two or more files. You need to map those into the below. To find out what the current data path is, in case the backup is from a different instance, you can grab it from:

EXEC master.sys.sp_helpfile;


Then the restore command is like this:

RESTORE DATABASE [new_name_here]
  FROM DISK = 'C:\path\file.bak'
  WITH MOVE Logical_Data_File_Name TO 'C:\some other path\some other name.mdf',
       MOVE Logical_Log__File_Name TO 'C:\some other path\some other name.ldf';