As to the specific events after Jedi, our heroes spend the night on Endor as the Rebel fleet protect the Ewoks from Death Star debris. Leia senses Luke is in turmoil and leaves Han Solo and the village early the next morning to find Luke and R2 miles away prepping the Imperial shuttle for liftoff. He says that he senses impending danger, though Leia states that there is no way that Imperial reinforcements could arrive so soon. Luke says that he was no match for the Emperor, and that his victory was actually a failure. He is going into exile to become a master, like Yoda.

It is then that the Imperial fleet attacks the planet, and the damaged Rebel fleet is no match for the Imperial forces. The capital ships are shot down, and the Ewok village is bombarded from space. Han Solo, Chewbacca, and Lando are killed in the bombardment, never having gotten to their ship, as is C-3PO (Han Solo never actually saw the Falcon again after his fateful statement before the battle). Luke, Leia, and R2 barely escape the destruction with their lives.

Leia is carrying the child of Han Solo, conceived in the first hours of Rebel Victory before his tragic death. Luke vows to rebuild the Jedi order and train Leia in the ways of the Force.

Some evidence for the "cloned Emperor, disreputable troops" idea:

The Emperor's "best" troops are babes in the woods, easily dispatched by teddy bears.

The inept Admiral Piett was stationed on the bridge of the Executor, Vader's flagship. Vader was instructed by the "Emperor" to return to the ship, but Luke's arrival on the planet disrupted this plan. The bridge shields of the Executor failed during the attack, to the seeming surprise of the officers onboard. This could have been a deliberate sabotage in order to ensure that Piett (and possibly Vader) were done away with.

The Imperial fleet is given direct orders to hold back and only keep the Rebels from escaping. After the destruction of the Death Star, they presumably retreat beyond sensor range to ambush the Rebels before they leave Endor.

Speaking of the Death Star, this time it has shafts running directly into the core, as if someone specifically wanted it to be destroyed in this way. The Death Star itself is honeycombed with cavities, saving on supplies while doing the bare minimum to make the station operational.

Vader only ever sees a hologram of the Emperor in Empire, and the hologram looks very different from the Emperor in Jedi. This hologram could show the true Emperor, with the clone decoy aging very quickly in the time since Empire so that he looked substantially different from the real Emperor.

The real Emperor, suspecting treachery from Vader, who failed to kill Luke after his attempt to turn him failed in Cloud City, chose to send a clone decoy to the Death Star to turn Luke to the Dark Side with the help of Vader. Whether or not Luke was turned to the Dark Side was irrelevant, as he planned for the destruction of the Death Star to make an example of the inept engineers who failed in designing the first Death Star. If against all odds the Death Star survived the Rebel attack, the confrontation between Luke and Vader and the "Emperor" would be instructive as a test of Vader's loyalty. If Luke was turned, he would have a new ally who could kill Vader, and if Vader turned traitor, father and son would not escape the Imperial fleet.

The "Emperor" brought dignitaries with him to the Death Star. These would be men whom he suspected of traitorous intent or ineptitude, and as the real Emperor was constantly in hiding for his continued safety, they would not recognize a decoy.