I do not think that Crete would actually have hurt Lucy. He would have found a way to make her at least convincingly pretend to believe his lies, probably by threatening or implicating her father's role in the disposal of Cheri's body and the death of "Joe Bill." However, I still think Birdy did the right thing and is the bravest person in the novel as Crete needed to be stopped. I think Birdy figured out that Crete was responsible for Lila's death (she would count the times he went by Lila and Carl's home when Carl wasn't home) and she had made a promise to Lila to be a grandmother to Lucy, so she kept that promise by protecting Lucy at all costs. I do not know what really happenned in the cave to Lila, it seems she could have fallen, and that her body eventually flushed out the back of the cave (like the dogs with the young boy, came out the back of the cave), but whether she fell and eventually drowned, it is still Crete's fault for manipulating her to go there and chasing her.