"As such Eragon, who supposedly grew up desperately poor to the point where the family was supposedly at risk of starvation, doesn’t come off as an actual poor person."

And after a few months of learning to read he's able to give speeches at the drop of a hat and doesn't seemed overwhelmed with the responsibility, new customs, opulent meals, and seems to settle into the role of being as powerful as a sovereign ruler.

Because he and the other humans seem to pop into existence he doesn't have any culture, customs of his own. Instead he absorbs whatever customs and culture of whomever he's staying with. Egh, I think I'm going to complain along with you how he doesn't seem to comment on anything and just rolls with it.