I'm high as fuck right now, but bear with me...

Not Indy’s degree. Screenwriter Lawrence Kasdan’s degree. Kasdan once said that “being a writer is like having to do homework for the rest of your life.” Appstently little Larry played hooky the day he wrote that scene. He could have had Indy add sand to the bag, and get the same result in a more plausible way. I’ve always thought that Kasdan is a putz.

PS Strictly speaking, Indy’s doctorate was in archeology.

I agree with almost everything you say, except the bit about the Mayans not having casting technology. They did, but they had not perfected it by a long shot. But if that figurine was solid gold, it would have had to have been cast, as nuggets that size are pretty much unheard of. A gold-plated wooden statue, however, would not have been very heavy. Sure, pure gold is very heavy, but Meso-American gold was full of impurities which would have made it lighter. And even pure gold plating doesn't add a significant amount of weight to such a small object, as the layer would be very thin.

More importantly, though, once the idol has been pilfered in such a manner, it would lose all value from an archaeological perspective. The value of archaeological artefacts lie entirely in their contexts, not in the materials in which they are constructed.

This. Seems pretty obvious to me.

Given that he makes mistakes even on this adventure, it is not unreasonable to assume that he has made mistakes on past adventures as well - learning from them. I don't think there's be any literature on Ovido photo-sensitive traps, for example, even in the Indy-universe.