There are many impact conditions established at address, such as...
Grip pressure point(s) which sense shaft lean and clubface direction.
Forward shaft lean.
Left hip set slightly forward and down which automatically sets the right hip slightly back and up. (We all know what that promotes in the downswing)
Shoulder lag promoting an in to out downswing.
Core muscle tension or awareness or activation.
I'm not going to go through all of those, but I disagree. Furthermore, I'll re-iterate that PGA Tour players have all kinds of setups, and they could all set up with "poor" variations of all of those and still hit the ball better than you (or I) on your/my best days.
I will point out that tilted shoulders (if you mean trail shoulder below lead shoulder) would conflict with "left hip slightly down and the right hip up". That'd be an awkward thing. Here are some more contradictions, and I'll add that the address shaft angle (forward shaft lean is rarely) the same as at impact.
This doesn't make sense to me. Impact is nothing like address. I don't understand what you mean by "establish good impact conditions at address."
I agree with the person above - EVERY good instructor "teaches impact." I fix the setup positions of a LOT of my students - likely a big majority - but I also fix backswings, because they too change impact, and arguable MORE SO than setup.
I rarely see a good setup. In fact, the basic fundamentals are often lacking which makes a good setup almost impossible to achieve. Most golfers who adopt a good setup position know what they are doing and I would suggest they tend more often to have good backswings and more solid impact positions.
Please define the basic fundamentals of a good setup. These fundamentals should be adhered to, by definition, by about 99% or more of PGA Tour players. I ask you this not because I think you can do it (you'd have to be so generic it's almost worthless), but to make my point: setup matters, but not that much (so long as you're set up within a pretty broad range you're probably fine, and other things matter a LOT more).
I agree that if you cast the club in the transition period, you're gonna find it hard to achieve good impact conditions. However as I argued above, by establishing impact conditions at address, surely a player's focus for imparting these impact positions has also been established at address.
That still doesn't make sense. I can put a player with a crappy swing in the PERFECT address position and his swing is still gonna suck. It won't improve that much.
I'd also like to point out that I'm not trying to say anything bad about your instructor, or any improvements you've had, etc. I simply disagree with the title of this thread (because backswing can AFFECT impact, and often does more so than setup, as well), and because as I've said ALL good instructors effectively "teach impact."