1. Cancel it? No, but if you don't take in some sort of recovery meal in between your weights and running you could break down muscle fibers and deplete yourself to the point where your body begins using muscle as fuel.
2. What do you think? What gets your heart rate up, a sweat moving and burning calories? If you're at an incline, you'll obviously go slower.
3. There's no such thing as spot toning. Less weight, more reps builds muscular endurance, more weight, less reps goes strength/size.
4. Why not do all chest stuff on one day?