Thanks for continuing to discuss this, Laughing Jaw. This case is one that fascinates the hell out of me.
So why didn't Buck turn on Stephanie? My suspicions are that (1) they were soulmates in psychopathy. Buck boasted to one of his fellow prison inmates that Stephanie would never talk about what had occurred on Palmyra because she was a "stand-up girl." What this meant for Buck is that Stephanie was as pathological as he was.
And (2) blaming the murders on Stephanie, even if they were her idea (as most of the participants believed they were) wouldn't have helped him. Buck had a deep criminal history -- that's why he fled to Palmyra in the first place. Stephanie joined him on th3e Palmyra jaunt because she was crazy and murderous, too.
Stephanie's first post-Buck boyfriend was another murderer, BTW.
Stearns is still alive, BTW, and she runs a senior center in Hawaii.