"1. It is possible to play a keeper as a defender. Not usually a good idea but nothing stops you. "
Not if you place him in sub GK. You can never choose another position for your sub-GK if you make the substitution by yourself. This is a bug because you cannot do it, but the system can do it. So, this should not be possible. This is a bug, or the rules should change and we could also substitute our sub-GK for other positions as well.