What bad things can happen with driver when you don't make a full shoulder turn?

Rushing the backswing and losing the grip at the top are great ways to destabilize your swing. It hurts worse on drivers and FWs because of their longer shafts.

A few years ago, I went into a local golf shop on a rainy day and bought some time on the launch monitor to check out my driving. I found that a smooth, full backswing really made a difference: With a swing tempo of 1.8 seconds, I consistently it the ball 15 yards farther than I did with a 1.4 second swing tempo.

Recently I've gone to a stack-n-tilt variety of swing, and I seem to do best if I just let my hands cock naturally at the top, focusing on steady  top position and avoiding "Zorro moves" at the top. That seems to be your problem in Q#2.

As for the 3/4 turn you mentioned.. Many middle HDCP golfers have a longer backswing than they think they do - I know this from my pro's observations, and from checking my top position on the tall mirror at the driving range. What feels like a 3/4 may be all you need, if you have quiet hands and can drop the club into the slot on the downswing.

Over-the-top requires the golfer do do what the pros call a manipulation, in this case compensating with hand and body shifts to regain and get the club back online. End result: it bleeds away direction and power.

At the top, consistent hand position and club angle will probably do more good than "reaching for a little extra." You just have to get the feel of top hand position and go with it. Ask your pro if he thinks this is part of the problem.

My thoughts, from someone who has two common swing faults:  not hinging my wrists (stiff wrists), and not turning completely away from the target.  If I can do these two things, I'll hit all of my clubs well.

If you keep your left (lead) arm straight through the backswing, which is a must for generating power and consistency, then there is really only one good way to get the shaft to parallel at the top:  turn your shoulders, rotating around your spine, until your back faces the target.  If your back isn't facing the target at the top, and your shaft is parallel, then you're doing something that is robbing you of power and consistency (grip is breaking down, arms bent wrapping around the body, etc.).

Nothing says you have to get the club to parallel at the top.  You can generate plenty of power with the clubhead pointed to the sky, so long as you're still rotating on that same axis, your left arm remains straight, and you get a good, solid wrist hinge.

Here's the problem I get into.  If you don't fully rotate your shoulders and spine (or if the arms and hands make it to the top, but you haven't fully rotated), then your hips are going to be way ahead of your club/hands at impact.  This causes the path of the clubhead at impact to be well to the left, causing pulls.  Combine that with the faults that got the club to parallel at the top (loosening the grip or breaking down the left arm).  If arms break down, you're not going to get the clubhead back to square.  Now you've produced a swing that is well outside-to-inside, and your face is open relative to the path.  The ball may start out towards the target, or even slightly left, but is going to slice and have very little power.

If I manage to keep the left arm straight and a good wrist hinge, but don't turn fully, my miss is always left.  Think about what usually happens (even with pros) when you try to hit a 3/4 9i or wedge into a green.

I try to think top of left shoulder to the ball. When I rush , its over the top and high floaters way right . I too am using basic stack and tilt .  So with the weight on the left side , its sometimes easy to try to rush everything to get to the way down , since I'm pretty much on the left to begin with .

Two things at fault when I'm not hitting woods well. Either less than full turn, or swinging too much with the arms and not enough with the hips.