Help me understand these driver adjustment results and possible shaft selection

It depends on what you mean by "while the core remains still" I wouldn't consider his core "remaining still" here, DJ's abs, belly button, and belt buckle have rotated almost 90 degrees by the top of the backswing. Those components of his core then have to rotate/uncoil on the downswing. Far from remaining still IMO.

It’s going to change but your basic setup feel should be about the same. Your body kind of adjusts to length and lie angle of the club. You’ll want to pay attention more to ball position as you change clubs more than how bent over you are. I understood what you meant from what your wrote.

Oh I probably was thinking the wrong thing or the wrong way when I typed the golf swing was more of a body swing (in my mind).  The in my mind part probably should have said in my opinion or based on what I have read.  Just meant the body needs to be involved more than the hands and swinging the arms while the core remains still.

The answer to this question is a very small change in impact can cause a BIG impact in direction. This is with the driver, but its similar with the irons... How much does an open or closed clubface affect your drives? | RoboTest Want to hit more fairways? The latest edition of GOLF's RoboTest highlights the importance of driver face angle. For a driver, 1 degree of clubface open or shut causes 10 yards of left or right. When you think the rate of closure for a golf club, how fast the club face goes from open (top of the golf swing) to square (at impact), 1 degree is a small quantity.  This means, when you are looking at video of your swing, you can see the exact same swing, but present the clubface in two widely separate positions. More so if you have swing faults. You can make the same backswing, but not get your weight forward well and hit a fat shot. You then react to this, try not to fat it so you thin it. Maybe the timing is off on the extension part of the downswing and you thin it slightly. Also, certain swing movements cause the wrists to flip at impact. The intent is still to hit the ball with the club. So your hands are going to adjust to find the ball. The more they have to adjust, the more difficult it is to repeat. Nope, it is way less mental than physical. If we could all think our way to a better swing, we would all be PGA tour players.  #GOLF IS HARD!  Beginners have repeatable swings, just bad repeatable swing. We all just need to make repeatable better swings.  The golf swing is a battle of making movements that allow the club to do good things. You results are from a bad swing that has a wider range of outcomes. Your outcomes are probably the following. Way more thins, fats, skulls than most. You hardly see PGA Tour players fat, skull or thin a shot.  The first principle of golf is FEEL IS NOT REAL. Meaning, we can feel things in the golf swing, yet it produces results or shows on video as something else.  You just set up to the ball. The club has a different length, so you need to adjust. For most your arms extend more out as you have a longer club. You also bend over less for longer clubs. It is just something you get used to. If you make a fist and stick your thumb out. You want that width from the butt end of the club and your belt buckle. Now, if you taller or shorter than most, then you may need to adjust the length of your clubs.    

At address? I think so. Downswing and through? IDK. I doubt if a whole lot of golfers, if any at all, deliberately think "I have a 9-iron so I need to bend half an inch more through the downswing compared to my 8 iron".