You're right; Lucas does have a screenwriting credit for Episode VI. And it's clear that the story is his; a screenplay is much more than dialogue.

There are lot of interesting blanks to fill in about what happened between the first film and the second. Why hasn't he left to pay off Jabba? I agree that it might not be a simple matter of leaving and coming back...and I suspect too that he's especially reluctant to leave Leia when he senses something significant emerging between them. Han's character has clearly undergone some changes already, but there is plenty of room for more growth, and that's plain enough from how he continues to mature in the final film.