Luther acting all shady while showing his wife the newspaper

I didn't think he was being shady about the paper at all, this isn't one of those movies where you need to read into things too deeply. Nora held the paper and read it herself, there was nothing to hide. The part where Luther says "it could have been anyone" was just a reply about who the paper could have quoted in regards to calling the Kranks actions "selfish". It literally could have been any of their neighbors, so Luther's response was appropriate. The only time he looks around suspiciously is after declaring that "they have spies everywhere" (in response to Nora asking how they knew Frosty was in the basement). Luther's suspicious glances are a reaction to the "spies" comment, as in they could be hiding in that room at that very moment, so he glances around suspiciously. I never got the impression that Luther planted the newspaper article.