People know the importance of the "something that only the killer would know" from police movies and other cases but on MM case this doesn't work and it's not that simple. Because "something that only the killer would know" must be something that police do know as well and no-one else knows. This apply when police do find a body, know how the victim was killed and no-one else except the killer knows.
On MM case we have PUBLIC AVAILABLE DVDs with PJ files on MM case that anyone can access and police don't have a body.
Even if CB did confess and stated "I KILLED MM" and stated for example "I stabbed her with a knife" how would BKA confirm or deny ? Relevance of "something that only the killer would know" only apply if police does know more than general public and know specific stuff that only killer would know. Also CB can tell "I took MM from the apartment when she was sleeping" so what, the entire world knows that as well so it's not something that only the killer would know and everyone with DVDs of MM process file can get a huge amount of details so this is not easy for BKA.
If BKA did had something like a photo/video of CB abusing MM it would be indeed something that only them and CB would know but that would be enough, they would present that to court and CB would be charged. If BKA doesn't know if they can charge CB with what they have then most likely it's not something that only the killer would know ...
Remember this is a thought experiment - it operates within defined parameters ----> that's the point
In my scenario, CB reveals the nature of the crime scene in a confession to an associate. Let's assume he reveals that he entered through/exited through the doors (no window) and was in the apartment for over 30 mins. In other words there are no checks.
This is something the real offender would know, that contradicts the public info.
The reason I find it an interesting inquiry is it potentially explains the apparent conflicts between the parties. In this scenario, HCW would be appealing to one of the inner circle to reveal critical evidence of the condition of the crime scene, which might corroborate his case.
I believe this is the direction Andy Redwood was also headed when he provided the offramp for Tannerman to focus on Smithman - he is not just saying a mistake got made. He is saying the public timeline is (for whatever reason) not what actually happened.