When a defender fails to conform to guarding rules, do you apply screening rules?
 

For example, if a defender establishes his initial position in the path of a dribble without having the torso face the dribble, do you apply screening principles on the defender?

Yes. That and the "each player is entitled to his/her position on the court proviede they get there first, legally" philosophy.

So in NCAA, can a defender obtain a legal screening position inside the restricted area to draw a PC foul?

If it's not a secondary defender on a player with the ball diving to the basket.