“I should have paid more attention to that sensation of still feels a little forced.”
The force that you would have had to counter was the impetus to be polite. In order to boldly follow your models, you would have had to tell the person on the other end of the chat that you didn’t believe his friend. You could have less boldly held your tongue, but that wouldn’t have satisfied your drive to understand what was going on. Perhaps a compromise action would have been to point out the unlikelihood, (which you did: “they’d have hauled him off if there was the tiniest chance of serious trouble”), and ask for a report on the eventual outcome.
Given the constraints of politeness, I don’t know how you can do better. If you were talking to people who knew you better, and understood your viewpoint on rationality, you might expect to be forgiven for giving your bald assessment of the unlikeliness of the report.
On #3, I think it’s more relevant to point out that many adults believe that God can make it alright to kill someone. What children believe about God and theft is a pale watered-down imitation of this.