Skinny skis definitely are easier on the knees- age, technique, aggressiveness, weight, all add to the formula.
Quicker turning for me is FUN- I can still tire my knees out by making a billion turns but
I found this article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4541126/ It has the photo from the original article-- and quotes from it!
Great article (and that is the original article, in digital form)! So nice when science agrees with your knees!
I just don't have enough experience skiing, especially powder, to know. But I have felt a preference for narrower skis when I happened to be able to compare. Last summer at Big Snow I went from 78 to 69, and the difference was phenomenal! (The 69 width were old Dynastar Glorys with a lot of steel in them, so they were also much heavier. But I liked skiing in them!) (And the 78s were the first skis I bought, Blizzy Quattros, and I was sick of them. My daily drivers (BP82s) spent last summer in Vermont.)