I think when you remove all the noise, which is about 80% - 90%, it is obvious and simple that the perp was someone who was very aware of the McCanns nightly ritual ie at tapas bar, patio door unlocked and children in bedroom/asleep. This narrows it down to the Tranmers. They may have taken her with a view to raise her as their own given they were a childless couple in their 50's. They could be forgiven for thinking the children were being neglected due to the McCanns leaving them unsupervised every night in an unlocked holiday apartment and thought they were doing all concerned a favour.
Maybe the mark in the eye made things difficult in that MM became more easily identifiable? Psychiatrist Raj Persaud makes a good point about this:
https://www.youtube.com/watch?v=0H0GXf1rto8Love the pictures he has in the background which are all sort of psychological
