And you're right, the opening of ESB is not set in stone. I think we can both agree that opening both ESB and RotJ with arrivals at the Death Star would be repetitive, so it's either one or the other. And as I mentioned before, there is no landing shot of Palpatine's shuttle. The original shot of his arrival is a squadron of TIE Fighters flying by the hangar, cutting to the shuttle sitting in the hangar full of troops. So the transition from the shuttle flying in space to the shuttle in the hangar would be too jarring. Even taking a shot of the shuttle flying IN the hangar from Vader's arrival doesn't seem to work. Also, I really think an Emperor scene would be most effective after Obi-Wan's final line, "...they could be made to serve the Emperor." That would have to be either this scene, or his second scene referening the fleet (which it wouldbe way too early for IMO).
Moving Luke's arrival to the beginning of the Rebel briefing works, with him hugging Leia then cutting to Han talking to Lando. There is then the problem of Luke not BEING at the meeting, and "I'm with you too." I'm not sure how to deal with that. One crazy idea I had would be for Luke to go BACK to Dagobah after Han's rescue and talking to Obi-Wan. Then would come the Emperor's second Death Star scene, followed by the Rebel Fleet scene and the movie would continue on from there. But I suppose that wouldn't make a lot of sense, would it?