I fail to see why the ending of a Roland Emmerich movie would be a Steven Spielberg contribution to ROTJ SE. The only Spielberg contribution I know to the SW saga is one or two ideas during the Yoda v Sidious fight. What are the others ?
As far as i know Spielberg was one the friends (besides Coppola, Huyck, Katz and others) Lucas consulted for at least rough cut feedback for the original Star Wars movie. According to the ROTS Bonus DVD, Spielberg was the one who suggested to George that during the Anakin VS Obi-Wan fight one of the antennas should break off and land on the lava river. Lucas also asked Spielberg for his feedback on the first rough cut of ROTS. Michael Kaminski hinted in his book ‘The Secret History of Star Wars’ that Lucas got the idea for the infamous AOTC conveyer belt scene from Spielberg, because Steven experimented at Lucasfilm with Pre-Viz shots for his movie ‘Minority Report’ (remember the conveyer belt scene in the Lexus factory) during the Post-Production phase of AOTC.