Stage 2: The Droids Play Hide The Lightsaber
Thanks to Lukes hologram message to Jabba and the all new deleted scene revealing Luke finishing off his new green lightsaber with the droids in attendance in a cave before the droids head off, we know the droids are sent to Jabba's palace by Luke. They roll in with C-3PO seemingly oblivious to any sort of plan and its soon revealed that their master Luke Skywalker offers them both up to the gangster slug Jabba the Hutt
As a token of my goodwill, I present to you a gift: these two droids, both are hard working and will serve you well. Seemingly Luke is doing this just to get a meeting with Jabba
I know that you are powerful, mighty Jabba, and that your anger with Solo must be equally powerful. I seek an audience with Your Greatness to bargain for Solo's life. This shows Lukes understanding of how the gangster world works but of course its fruitless, largely because Jabba doesnt believe Lukes claim that he's a Jedi anyway.
So Why Are The Droids Actually There?: Luke sends them in as a bargaining tool. So it seems the droids are sent in to test the water. Maybe Lando has witnessed this bargaining approach working with Jabba previously and suggested it might work in order to get negotiations for Han Solos release underway. Either way, were now at the stage where Lando is still undercover with no end game in sight and now the droids are taken hostage by Jabba the Hutt. Arguably, if this bargain had actually worked, Luke didnt give any instruction on how Jabba would arrange a meeting with Luke, so we can assume Luke always intended to just show up a couple of days later. This is backed up by the fact that Luke now has R2 inside the palace and inside R2 is his lightsaber. This adds another layer to the plan but also adds another question: Why did Luke need to hide his lightsaber in R2-D2 in the first place? Just wearing it on his belt might have made more sense, right? it's not like he was searched on his way in or anything. Anyway, the long winded rescue plan continues.