Decrypt T-SQL log backup header and read LSN
Could anyone confirm or denies that restoring the header only wouldn't
  affect the sys.fn_dblog or anything else?


RESTORE HEADERONLY doesn't target any database in its syntax:

RESTORE HEADERONLY FROM &lt;backup_device&gt;   


It reads the backup device and returns data to the client about what's in the backup.  It won't affect your databases in any way.