That hits on another problem if the message is never delivered how do the droids get into position to play their part (not that Threepio does much) in the rescue?

Jawa's snatching droids is one thing but I can't believe Jabba or even his cronies would would do something like that.

Sticking the Emperor's arrival so early it does cram in even more exposition at the start and there already is a fair wack but the dialogue does seem to demand it (point taken about keeping the acts seperate but we got a lot of cutting back to Tarkin in ANH and that didn't hurt much).

My instincts say.

Yoda or Vader first (and it's really much of a muchness which) something that could be weighed up by doing a dummy edit.

Yoda or Vader second (depending on what happened first).

Then Luke on Tatooine (in whatever form is possible)

Then the Emperor arriving, then droids, then Boussh, then the rescue, the briefing, (for those who want the Boba Fett plot part one goes before Leia revives Han but after Chewie gets dropped off, Part two takes place just before the briefing).

Having Yoda first though means that you don't get two Luke scenes right next to each other (you do get a evening followed by a morning with the droids but ANH had that sort of thing anyway).