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 ?
Well I believe Spielberg said that he watched a rough cut of the original SW with a group of George’s other filmmaker friends, and he was the only one who liked it. So I’m pretty sure he’s been giving George encouragement for a long time. Not sure about specific contributions, but I find it hard to imagine he’s never tossed out ideas while talking to George or that George wouldn’t take them into consideration.