What would happen if the Empire managed to successfully escort Leia,?
Leia; almost certainly tortured. If they realized she was Force-Sensitive, then they'd have tried to break her, brainwash her, and train her as a dark side adept a la the Inquisitors, possibly even a next in line to Vader. If not, interrogation for information on the Alliance, then execution. Chewbacca; hard labor or execution. The empire was xenophobic in the extreme (there's a reason Thrawn, being a Chiss, is unusual). Other possibility MIGHT be using him as bait for the Alliance to attempt a rescue, since he and Han had friends. Threepio; depends how sentimental the part of Vader that's still Anakin feels whether or not he gets scrapped after scouring his memory banks for anything he knows about the Alliance's plans.