You’re confusing the ideas for a low-budget sequel, which George Lucas considered while making the original film (in case it performed poorly at the box office), with the much more expansive ideas for a Sequel Trilogy, conceived during production of ESB.
In the former case, Lucas was genuinely afraid that SW would be a flop, and took measures while writing the 1976 fourth draft to make the film work as a standalone story if necessary: by doing things like introducing Tarkin as a proxy character for the Emperor, whose death represents the Empire’s eventual defeat.
However, he also talked to Alan Dean Foster about doing a novel or two, which could be used as the basis for a couple of TV movies in case fully-fledged cinematic sequels weren’t possible. Those discussions led to Splinter of the Mind’s Eye – which at the time was conceived as being a potential capstone to Luke’s journey. Hence Leia survives her duel with Darth Vader, but is badly scarred, while Luke kills Vader and thus avenges his father’s murder.
(One reason Lucas was panicked about not getting the chance to do sequels was that even back in 1975, he’d already envisioned SW as a trilogy, plus at least one prequel film: echoing Richard Wagner’s Ring Cycle operas, a “trilogy with a prelude”.)
In the latter case, the massive success of SW 1977 led Lucas to start more expansive planning for sequels, and hitting upon the idea of a trilogy of trilogies.
It seems that at that point, Lucas wanted the film after ESB to feature the defeat of Darth Vader, but also the demise of Han and Leia. Probably this was a result of the chemistry between Carrie Fisher and Harrison Ford; Lucas had wanted Luke to “get the girl”, but real life moved events in another direction.
Lucas mentioned in the newsletter Bantha Tracks that the Sequel Trilogy would focus on “the character who survives Star Wars III”, almost certainly meaning Luke. As we also know, the plan was for the ST to focus on Luke’s adventures with his long-lost sister, and their defeat of the Emperor.
Boba Fett being the agent of either Han or Leia’s demise in “Star Wars III” probably figures in to what Craig Miller meant when he mentioned Fett’s expanded role. One possibility would’ve been for Leia to be killed in battle by Boba Fett’s disintegrator: turning her into dust, like the planet Alderaan she was princess of.
