When the Met ruled Tannerman out they did it by accepting that someone was returning from the night creche when he was actually heading towards it. They also accepted that someone kept the clothes they and their child wore that night for six years and remembered which they were. Hmmm.
If they were as meticulous when examining the parents and their friends it's hardly surprising they managed to rule them out.
As ruling the parents and their friends out also rules out accidental death, there's not much left to consider when using the 'balance of probabilities' test. Either MM left the apartment herself or someone took her. Once you've excluded involvement by the group you can rule out woke and wandered because they said they found all gates and doors closed, which makes that theory unlikely. Because you've ruled out involvement by the group anyone taking her was a stranger. The probability being an abductor or a burglary gone wrong.
The problem is that circumstantial evidence exists which suggests parental involvement.