In reply to each numbers in you last post:
1. Going with simple anatomy you were referring to.. why would the same motion of the humerus would result to a different promotion of muscle activity if muscle fibers are either fully promoted or not. Flies concentrates the load on the chest since the triceps are not helping with the lift.

2. and 3. Where is the evidence? I don't see any citations of peer-reviewed articles. Yes, muscle do stretch and contract from the insertion and origins points. But, when I do a 'one arm' press from incline, flat and decline, I feel the same contraction from my chest area .The only way I was able to differentiate was with significantly lower weight.


4. Skipped, not involved in this one

5. If I read correctly, you wanted to argue (''didn't want to not argue'')

If I resume the above, it is not BS. Genetics is the biggest factor, lift heavy on any of these exercise and the chest will certainly be worked all across.