I would never share an account. With nobody, so "sharing" it is out of option. As for "difference in English proficiency" it all depends which "mode" I'm in at the moment.
How do I know I've lost some of my stuff? Characters' inventories had holes and some crucial items I've had remembered to have were lost... along with a fat purse and a teddy bear... So it's logical to assume that somebody just got into my account and took what have had any real value. That's it.