I'm intrigued by your comment that John Knightley was being mischievous in describing the snow. I've always seen this as plain crabbiness, but he certainly does attack Mr. Woodhouse unfairly about it. ("This will prove a spirited beginning of your winter engagements, sir. Something new for your coachman and horses to be making their way through a storm of snow." - from Ch. 15). And I can see that the crabbiness that allows him to be so rude to his father-in-law (in punishment for making him go out when he didn't want to) might also lead him to punish Emma's obtuseness by leaving her alone with Mr. Elton.