^ That was for reasons of plot security on set, rather than that detail not being decided by shooting time. The scripts also don’t mention Luke having his hand cut off and replaced with a prosthesis. (Earlier drafts call for special insert pages at those particular points.)
Yes, with the amount of secrecy involved, I can believe that it was planned but deliberately left out of the screenplay. Same with the Leia reveal in ROTJ.
One thing I have noticed from reading the earlier drafts of the original movie, is that things tended to carry over, even if they went to different characters. So a lot of the details change, but a lot of the core elements remain. Twins was one of the recurring ideas so I can see GL toying back and forth with the idea. But aren’t Luke and Leia supposed to be the same age? I know Hammill and Fisher aren’t the same age, but the characters were supposed to be even back in 1977. How set GL was on the idea I couldn’t say, but I think he left the story open to that option.
From design ideas for Jabba, I think it is clear that Jabba was designed for that scene with him in ANH. I can’t say if that design came from 1977 or from 1983, but I think it is clear Lucas was still thinking about the cut Jabba scene when they were designing the way he’d look.