I agree with JP. In all 3 situation you described, it would be best to begin resuscitation, head for the truck, and let the hospital deal with the family.
I have always been told that if the family requests efforts be made, you go ahead and start CPR, despite the DNR. Also, if there are ever any doubts about the DNR or it's validity, start CPR. (This it the policy of my department, mind you.) Especially, if the scene is deteriorating and your safety is becoming an issue. Sometimes, the circumstances dictate that you have to make decisions your wouldn't ordinarily make.