DC, your first paragraph is correct. The circle would not have mattered on that throw in,but that does not change the fact that David did indeed run over a legal screen.
Paragraph two: On a back screen where the defender can not see the screener he MUST give the defender one step. NOT so on a screen to the side or front. He must maintain legal guarding postion. We do not do a very good job of setting screens and haven't for several years. The defender can not be moving sideways when the contact occurs. He must be in legal guarding position and you are not when sliding and contact occurs. In your second and third situations you should only see a foul if someone gains an advantage. Most of the time an advantage is not gained ,hence no whistle. I agree with you, there shouldn't be whistles on all contact but there should be where someone gains an advantage. MOst of the time when the defender comes into contact with the screener a switch is made or he rolls off. Only time a whislte should occur when the defender runs into an offensive screener , is when he runs him over or clearly gains an advantage. Did we gain an advantage last night when the officials chose not to call David for running over the screener? We sure did,but we'll take it. Hope that answers your questions.Thanks zebras