JM was able to put BB at the scene. That, and it would be difficult for someone to pull this off when English isn’t their primary language. She doesn’t have that same “sophisticated” sense like BB does. He is use to being ambiguous and savvy with his job description.
Even with that, he missed some important points to not become a suspect. No Burner phones, no activity on the Fet site after the deaths, posting photos with his fling versus a true grieving husband and more.
He doesn’t realize how guilty he looks in court. The fake tears on cue whenever Christine’s trauma is revealed isn’t working for him. As soon as something is said that is really uncomfortable for him he wipes his eyes. It’s robotic. He can’t pull it off.