One more perspectiveI'm new to this site, so thanks to all the insightful, passionate participants. I'm learning alot reading all of your comments. I have less expertise, but I am practical. My background is multi-sports, now coaching Junior Club level racers. At my club, we've stopped "teaching" shoulder angle, prefering to work on dynamic balance and powerful movements (not in words, in actions) on varying terrain, and let the shoulders tip how they will, ie, more in favorable conditions, and level-ish in sketchy conditions.
For D(C), who started all this, here's what I'd say if he was in my program: first, let's check the equipment. Fess up--are you in macho stiff boots and skis that the big boys use but you can't really flex? If so, then stiff back-seat steering is your fate. Maybe the straight arm problem is trying to push forward against stiff boots? Second, in the pics and your comments, you seem too analytic and tight, typical of adult learners, but seen in kids too. Not only does your equipment need to flex more, YOU need to flex more in body and mind. Quit trying to find "positions" and the "right way" and strive for balance, flow, feel, athletic movement. Loosen up and take risks, do the drills that are suggested here and by your coach to their EXTREMES and have fun! For what it's worth, here's my favorite drill progression for your issues: without poles, on a nice groomer, ski down with your hands on your knees pushing them symetrically from side to side to create your arcs. When you are good at that, put your hands out comfortably to the sides and keep the knees going as if your hands were pushing them. DONT TRY to level or tip your shoulders, just let the terrain determine the tilt. Use your hands for balance, DONT force them into a position. When you're good at that, then EXPERIMENT with more shoulder inclination or level shoulders so you learn how it feels and works. Sometimes you'll look "old school" and sometimes you'll look like a snowboarder--all good!!! This is big fun and helpful too.
Thanks again for the great discussions everyone.