LEFTJOINの注文と制限

交換してみてください：

LEFT JOIN ps a ON a.account_id = fr.author

と：

LEFT JOIN ps a 
  ON a.PrimaryKey                         --- the Primary Key of ps
     = ( SELECT b.PrimaryKey 
         FROM ps AS b 
         WHERE b.account_id = fr.author
         ORDER BY b.level DESC
         LIMIT 1
       )