In fairness, I think it was The Force Awakens that painted the films into a corner, if anything.I don't disagree TLJ painted this final film into a bit of a corner. I mean, JJ pretty much admitted it. I'm pretty rare in that I'm not on either side of TLJ. There are parts I hated, but there are parts I thought were really cool. And I kind of liked how Luke was handled. I do think it was wrong of them not to rework the film a bit when Carrie died, but that's a whole different thing.
The so-called mystery of Rey's parentage? Many people wanted a big reveal like she was a Solo or a Skywalker, but that doesn't really fit with what TFA showed us, unless everyone involved was not telling her the truth, to force her to find her own way. Heck, it was fans pushing such theories, not the movie, which made a point of Rey being "no one" and establishing that her answers lied in the future, not in the past, and not from parents who aren't coming back.
Luke going into exile because of his failure with Ben Solo was again, something a lot of people didn't like about TLJ, but again that storyline was established by TFA.
I liked what TLJ set up for the final film. The heroes reduced to a scrappy rebellion gives them a challenge to overcome. The notion that the force belongs to more than just the Jedi. These elements set up a scenario where the greater galaxy can come together in opposition to the First Order.
I don't know how or if adapting to Carrie Fisher's death would have worked. The filmmakers are on record saying there was no way they'd cut her final appearance short, and certainly not cut the reunion between Luke and Leia. I agree with them sticking with the story they had, even if it added some complication to making Episode 9. I would have been fine with recasting Leia, in order to finish her story and the trilogy story as intended.