As far as that mechanic…
As far as that mechanic saying he has never not one time seen a serpentine belt fail at less than 75 or 100k miles.... That's just pure nonsense. While it's true that serpentine belts don't "usually" fail early in a vehicles life, there are many reasons why they can and do. If this mechanic has never had to replace a serpentine belt on a newer car I suspect he is a very new mechanic. As for the rest of the trouble on this Silverado, it's inexcusable.