In season 1 episode 5, during the parent teacher conference, we see that Stiles' real first name ends with -ienim, as shown in the picture on this page. However, when revealing Stiles' real name in season 6 episode 8, Sheriff Stilinski said "I have a son. His name is Mieczyslaw Stilinski"... Does Stiles have a middle name that we are not aware of, or was this a overlook?

True that.