And FWIW, in early stages especially, a "what if" guilt can overshadow thought processes. It's possible that both JI and SB are feeling lots of "what if I came home on time Lisa would be here", "what if I didn't drink with her", "what if I paid more attention to things"...
I had brought that up early on. If DB was truly innocent, there are serious levels of guilt that one must go through and I wouldn't doubt that she went through them and perhaps is still going through them.