I’m no expert in human behaviour, but I’ve been involved in criminal law long enough to have a bit of an understanding of motive.
None of this really makes sense to me either. I just can’t understand how the prime suspect has been sitting under the noses of police this whole time. It’s not normal that a person could be interrogated by police over and over again and not give up any inconsistency altogether.
IF the FM is involved, the only theory I think that makes any sense is that there was an accident, a panicked coverup by one person, non disclosure to ANYONE (except maybe the FD who witnessed) and then sheer dumb luck that the initial investigation pursued other leads and lost valuable evidence.
and IF that’s the case (very big IF) then there must have been some sort of psychopathy that enabled her to believe her own story so that her story became her truth.
That’s just my theory.