Oh ho ho. Well, you have piqued my interest. Realistically, Mary's going to Scotland because there's no way in all of hell that Anne is letting Katherine's daughter get France when she wants that for her kids.
As for killing off Katherine - yeah, the sweat can take her easily enough. Remember, in 1528 Katherine is 43. By Tudor standards she is ancient so no-one would be surprised or shocked if it did finish her off. Hell, Henry could even use her death to bolster his marriage to Anne, especially if the divorce proceedings have begun: a sort of "See, now she has died of the Sweat like her true husband, my brother Arthur."