I would like to settle this: Swinton was infinitely more surprising a snub. She had everything and the category seemed set. Nobody cared about Mara throughout the awards season, and she somehow was nominated without the film in Best Picture. It is truly the most bizarre nomination I can think of in recent years when you look at how the film performed. Sure, Kevin's grim. But Swinton campaigned(!), went everywhere, and actually had a campaign set for her. The snub makes sense in some ways, when divorced from the reality of the year and the limited options.
Meanwhile, aside from Lawrence and Chastain, this year's category was always in flux, and Riva, another foreign language performance, was in contention for a much better seen film. To me, she was never in as strong as position as Swinton ever was, mostly because of the category's competition this year.