I think Kennedy was a disaster, but #3 would have been considered “taking initiative” in most combat zones. As you state later, the cannon was lightweight. There is no info that it had any bearing on the sinking.
As already stated, early PT boats were lacking in gunfire for attacking barges and light boats. To the point that some were retrofitted as gunboats, no torpedoes, one of which Kennedy commanded after PT-109.
I can’t remember if it was the Japanese version or the US Version of the 37mm. In any event, a fully mounted Japanese version was over 700 pounds, the US version over 900 pounds. The actual weight on the PT boat would have been lower, as they left the wheels off when they dropped it on the foredeck. Of course each of the really crappy torpedoes it carried were much heavier, but they were mounted per the boat’s design.