wow, I think I struck a nerve or two. Ok, so lets break these down.
1. Yes, a muscle will be more defined with less fat around, that's a given. However, the muscle fibers on the edge on the muscle(with pecs i mean where it meets the shoulder and the rectus abdominis) are forced to fire when the muscle is stretched and then contracted. So with flies, you stretch the edges of the muscle then contract, or bring the weight across your body. Muscles are build of fibers, fibers only grow when forced to work more than they are used to. Simple logic of anatomy.

2. I do have evidence that incline and cables being pulled up and across your body work upper pec more than lower or mid pec. Look a muscle striations. When it comes to the pec, the striations fan out. Striations are built of fibers and fascicles. They get longer in a stretch and shorter in a contraction. They can only contract in one direction, which is why you have a pectoralis major and minor, they have different directions of striation relative to each other. So, if you look at how the muscle works, and you move your arms, you will see that presses and flies at 30-40 degrees(any higher will be more shoulder than anything else) will cause the upper pec to fire more than lower and mid. With cable, the direction affects the portion of the muscle fired. Take your right arm, reach across your body at the waist and feel your right pec, then raise your arm to level with your head and the left side and feel the muscle.

3. Yes when you work a muscle, the bulk of the muscle will fire, I'm not arguing that. The more the decline the more pec isolation you will have, and the more incline the more shoulder you will add into the lift. However, the upper pec fibers will not need to strain as hard to lift with decline, you will use more mid, lower, serratus anterior and lats. Next time you do decline press, use 1 dumbbell and at full extension touch your pec. The lower pec will be tight, while the upper pec will feel softer.

4.Yes, I know that that could be considered "begging the question". However, I didn't talk about something different for my argument. I talked about muscle fibers and how they work different when altering a lift. Same thing, different muscle group. That doesn't change facts.

5. I learned fallacies in Philosophy 101...didn't want to not argue with you and cause you to feel left out. hahaha.

So, as i stated before, not being able to isolate upper, mid, and lower pec is total bullshit. And as always, Good luck!