Ray Park didn't do the fight choreography, Nick Gillard did, and he continued in the following two films.

Captain Panaka dude wanted too much money to come back. This is what I remember TFN reporting back in the day. The Star Wars films have never operated like normal films would--where a returning actor likely would be paid more. So they wrote him out and replaced him with a character that is...his brother or something? I forget what the explanation was. But he looks and acts similar, except he isn't the same guy, technically.

Interesting story about David Prowse. The story I've heard is simply that he gave away a couple seemingly harmless tidbits, but he wasn't even supposed to be doing that. He does have an ego and is also a bit self-promotional, and I guess he just got on Lucas' bad side.

Personally though--what's the point of all the secrecy when you release the novelization of every single Star Wars film in April? People who want to know all the secrets will have been given them by Lucasfilm 6 weeks before the films open.