Well it's hardly a credible scenario is it? After being pissed off in heat of the tractor, he decided 'it's tonight' - and the hitman just happens to be ready to go at a moment's notice.
I'm not sure where you are heading with this, it's pretty clear that there are three main possibilities relating to Julie coming forward. In no particular order:
1. Julie is just telling lies, either off her own back or coerced by others.
2. Whilst JB had told her about the murders, she didn't believe him and thought he was making things up, even after the incident.
3. She knew all about it and was happy to stay with JB anyway.