If I set my lead arm connection and bend from the waist, my lead arm will be where it will be right?
I think even with connection you can still have it be steeper, or flatter. After all I think it is steeper with your irons.
As long as I maintain my angles, bend and connection, what difference does it make how flat or upright the lead arm plane is?
The flatter the plane the more your arms will move away from your body at impact. Compare to a baseball swing and you will see what I mean. I don't think it is wrong, unless you want your trailing elbow against your body at impact.
It seems that this relationship is dependent on address - forward lean and length of club. The real question is what is the benefit to having the lead arm swing on a more upright plane?
It seems that all pga tour players are steeper then you. That still doesn't prove that there is a benefit though.
The more that I improve my own impact position, the nearer I need to stand to the ball at address, which makes my leading arm swing on a steeper plane.
There could be a benefit to flatter. I still like hitting some balls SA. It seems easier to hit the ball straight, but harder to get an ideal impact.
As far as the shoulders are concerned I think the pictured players trailing shoulders may be closer to the ball at impact then yours is. It would be best however to look at shots where you are hitting a ball. Maybe you can take a halogen light with to the range next time you go.
I see your point with the shoulder tilt.