Do we know at what point in production the ‘darker’ script with “Had Abbadon” (the Imperial City) was jettisoned for a ‘lighter’ script with re-use of the Death Star instead?
Just to be clear, the Death Star was always there in ROTJ. In fact, there were TWO Death Stars being constructed above Had Abbadon!
That said, they weren’t as far along in construction as the one in the final film and I don’t think there were any scenes set aboard either of them. As well, there’s no need for fighters to enter them as their cores are exposed enough that merely bringing down the shield is a sufficient weakness for the rebels to exploit.