I know Lloyd Alexander wrote a lot of other books, but I don't think I've read any other than the Prydain series. It wouldn't surprise me a bit that he didn't know what to do with a clever teenage girl.

That's an interesting thought about what other characters could have risen to the challenge of taking over Boone's spot as The Hero. Sandoval certainly, if he'd had a reason to. Lili would have been amazing. There was a virtual second season written back in the day (by a bunch of writers, I believe) in which Boone survived and Liam didn't take his place. Liam was there, but I don't quite remember how they worked it out. There are also some series I've come across where Boone and Liam are both Da'an's Protectors. In one of them Boone is aware of Liam's parentage, but Da'an is not.