Yes, your first I is wrong. I think 4dn is explained earlier. It's a themed clue, the pairing of a common word for "people" in the plural, with the "side swapped".
I'm glad you've got 12ac. I can't add anything more to the parsing of 4dn, but if you've got all the crossers right, there aren't many words (it's a common plural) that will fit the grid.