P4noz, I understand what you are saying, but I think you may be missing some factors.

1. Yes, with flies you can't do as much weight because the triceps are not involved. However, the humerus doesn't move the same with presses as it does with flies. To prove this, bend your elbows like you would do a press, then move your arms like you are doing flies. The motion, direction, angle all change. Its subtle, but there is a definite change. That's the second biggest factor in the difference between flies and press.(the first being triceps not being involved as much in flies)

2 and 3. Quoting other people proves nothing. I could quote people all day about the subject, but I would get the argument back of "thats their opinion, and i feel they are wrong". So, its pointless to do so. The reason you feel it more with lower weight is the other fibers are not used as much because you don't need to compensate to lift the weight. Don't you think that if you feel a difference at lower weight, that it would mean that there is a difference. Just because the bulk of the muscle is forced to fire along with the focused portion doesn't mean you aren't hitting the focused portion harder than with other lifts, it just means that along with causing it to work harder(which results in growth) you are using surrounding fibers to help move the weight. Its logic.

4. I too shall skip since there is nothing to talk about.

5. Yes and no. I didn't want to argue with him, because all he did was comment on another persons argument. However I didn't want him to feel left out, since I argued with everyone else, so I argued with him. It sounds redundant, but it is technically correct....confusing...but correct.

Now to comment on your final thoughts. Genetics does play a large role in body building and muscle growth. However, we are not bound by our genetics, (unless we have a muscle deterioration disease or something). You just may have to work a lot harder for it. Some guys just seem to explode, others not so much, but given enough time and work everyone can achieve a large, muscular physique. That's why there are sites like this with people giving various tips and tricks for people to try to see what works best for them.