To make it have no rotation you will need to add another variable to the Sprites and call it "angle_toMove". Then istead of setting the angle of the sprite set that variable to angle(self.x,self.y,self.target.x,self.targety). You will also need to change the two "move forward" actions to "move at angle" actions, where the angle will be self.angle_toMove.
I don't understand the isometric portion of the question, can you expound on what is different about isometric movement?