Wow Andrew! That's amazing about the 17 mountain ranges. You're right that it's probably just coincidence, but it's an awfully cool coincidence.
About people reading the postcards... Well people are nosy and it's a tiny town. Plus, Alma reads Jack's postcards, we know that. Jack and Ennis were clearly almost counting on the idea that people were going to be reading the postcards... thus the elaborate, long-lived stories about fishing trips.