It felt to me like George Lucas was being given too much of a free hand, and so there wasn't anyone doing that checking

FWIW, this is pretty much the story that I got from reading (ironically, I've forgotten which) Kaminski's The Secret History of Star Wars or Marcus Hearn's The Cinema of George Lucas: Lucas, for the original trilogy, was able to circulate drafts of the movie scripts among a circle of friends and acquaintances, who would edit and make suggestions and criticize it heavily. This circle had, however, disintegrated by the time the prequels got going, between normal life events, Lucas's divorce, etc.

(As far as Empire Strikes Back goes, you can't give any of the credit to Leigh Brackett; she was dying at the time, and supposedly Lucas wound up ignoring what she managed to finish.)