At least in Canadian English, I don't believe that it is used with respect to a person being guilty or innocent of something. Given the choices in the poll, however, "looks innocent" would be the best of bad choices IMHO.
To me the idiom "butter would not melt in his/her mouth" is more of a comment on a person's character in general. Someone who is very pure, never swears, never drinks or goes to bars and is universally pleasant and kind in every way, may be characterized as someone for whom butter would not melt in their mouth.
This is not particularly a flattering description. Often it is said with a sarcastic tone, often by someone who is jealous of that person for some reason.