Luke's plan, as I see it, was this: A) Lando gets hired on as one of Jabba's guards. Since Cloud City was overrun by the Empire, he could have told Jabba he needed some short-term employment. Lando's much smarter than a Gamorrean, so if he worked for a price similar to a normal guard's, Jabba would jump at the chance to hire him. Han was frozen at Lando's Cloud City, which is a definite sign - in Jabba's eyes - of Lando's dislike of Han. B) Luke sends the droids to smuggle his lightsaber in and try to barter with Jabba. If Jabba decides to give up Han, then Luke has saved himself a lot of trouble. All he has to do is contact Lando to get him out of the palace. C) Since Jabba doesn't want to give Han up, he sends Leia in. Chewie serves as her reason for showing up, and once she's in she can presumeably mingle with the crowd under the guise of finding a new bounty. When she gets a chance, she can free Han from the carbonite. THAT is the reason why Leia went into the palace. Moving Han is much easier than moving a block of carbonite. D) Luke himself shows up. He tries for the last time to convince Jabba to give up Han. If Luke's mind-trick works, the entire group can just walk out. But it doesn't. So he does something to really get Jabba angry (kills the Rancor) and gets him to send the whole group out to the Pit of Carkoon. (Presumeably, this is a common occurence with Jabba. His sail barge seems well-suited for watching executions). That way, he doesn't have all the defenses of Jabba's palace to fight his way through. It's a pretty good plan, IMO.