"Server not open" / "Drop tablespace" catch-22 situation
No, it is not circular, alter database open is the 3rd step.

If you have a backup of that datafile, then restore and recover it.

If you do not have a backup, then do what the post you referenced tells you. Since your database is already mounted, you can skip step 1.


  
  If the database is down, mount it.
  
  
  STARTUP MOUNT;
  
  
  Offline drop the datafile.
  
  
  ALTER DATABASE DATAFILE 'full_path_file_name' OFFLINE DROP;
  
  
  If the database is at mount, open it.
  
  
  ALTER DATABASE OPEN;
  
  ...