Does divorce occur and a divorce pronouncement (talaq) count against a man who said to his wife's brother: "Her papers will arrive after forty [days]," intending to scare him and end the conversation, and not intending divorce, knowing that his wife was in postpartum confinement (nifas)?

What you wrote to your wife's brother does not entail her divorce, nor is it counted as a divorce. Divorce only occurs through an explicit pronouncement or an implicit one accompanied by intention. The mentioned phrasing is not explicit in divorce; rather, it is a promise of it, and divorce does not occur merely by a promise.