Jane sees so clearly. She sees Rosemund and her loveliness, she sees StJohn and his priggish will to do his missionary work. Maybe he does see clearly that in a year with Rosemund they would be unhappy, but I think Jane still wishes him happiness.
Amazon’s cap -Amazon means riding cap, so a soft velvet cap, similar to what we would call a beret or tam-o-shanter. Sometimes the tops were almost like a mortarboard but taller than we traditionally wear now. Amazon often meant female horse rider, thus why Rosemund was wearing it.