Or if she hadn't tried to cram so much material into the last section?

That couldn't have hurt. Especially considering the leisurely childhood opening, the breakneck of the tragedy is pretty zero-to-KABOOM. The stuff with Guenivere is also really interesting, but just barely sketched.

the quartet goes out with a whimper instead of a bang.

I'll try to find the interview with her where she explained that she didn't mean to write an Arthurian quartet, The Crystal Cave was supposed to be a standalone, but then she just kept thinking about the stories.