Lack of rehearsal time could explain a lot. Fairchild studied a lot of jazz growing up.
Things can change when a dancer really focuses on training classically, the way an opera singer who once studied pop music can lose some of his feel for pop style. Quiggin's phrase "sense of the vernacular" was what I was trying to get at (Quiggin just said it better). Kelly studied quite a bit of ballet, but iit wasn't his first dance language, as he would have readily admitted.
Yes. Fairchild has a longer mid-section which throws the center of gravity off and so the waist has less importance
None of this may make much difference in the new version of "An American in Paris," which will be staged by a ballet choreographer and danced by two ballet dancers, which may shift the show's balance decisively toward ballet.