Say Luke Skywalker (Jedi) is turned to the dark side. Then what happens if the Rebels then win a combat with both Luke and Palpatine/Vader in the same system? Can the Rebels kill a Imperial leader? Like, say, Luke?

Based on RAW,yes. Bolstered by his former comrades' victory, a flicker of good emerges in Luke. But realizing he's gone too far to the dark side to return, he ignites his lightsaber and ends his own torment.

Or aspiring to greater mastery of the dark side, he uses the confusion to eliminate one of the other Sith to find himself one step closer to being the master.

Luke could fulfill ESB Vader's hope of defeating the Empire and ruling the galaxy as father and son.

(I've always called this "the best possible ending".)

Yes, yes they can.

All the following scenarios contain a turned Jedi Luke Skywalker.

Luke and Vader. The Emperor convinced the more powerful Luke to kill his father and take his place. Luke attacks and kills Vader. Or Luke attacks and Vader defends himself and kills his son.

Luke and Palpy. Vader convinces Luke to kill the Emperor so they can rule the galaxy as Father and Son. Luke attacks Palpatine and kills him, or Palpatine counters and kills Luke.

The battle that allowed the objective is just the catalyst to the situation. Luke was looking for an opening, and the battle created the opportunity for him to strike.

Interestingly enough, the card would work with a captured Jedi Luke, or a Carbonite Jedi Luke. Luke isn't the one doing the killing, he's just in the system. Maybe the failed battle drives Palpatine to kill Vader, or when Vader goes back to retrieve the frozen or captured Luke, he gets cut off from his forces and gets eliminated by Rebel forces.

Well it's unsurprising it would work with a captured Luke. He was captured when he did this in the movie. He surrendered!

Response from FFG

A Sith and his apprentice turning on each other with one killing the other?

That NEVER happened in the lore :-P