Abduction is the simplest and most plausible explanation in this case, with "died before the 3rd, and death covered up by both parents and half a dozen of their friends, aided and abetted by senior members of government and UK police" probably the least likely - all in my opinion of course.
Don't forget the nannies are either mistaken, or have been bribed. Anyone who says they saw Madeleine are wrong.
They are calling all witnesses who say they saw Madeleine as being liars in other words. Why on earth would they say they have seen Madeleine if they haven't? Surely they would say they weren't sure rather than say they did, they were speaking to the Police after all.
I find it quite arrogant that they dispute these witnesses and what they say so that their idea of what happened to Madeleine can be shown to be plausible.
Kate went to the crèche and didn't take Madeleine, even though the nanny said Madeleine came out pleased to see her Mum or Dad and Kate was eager to talk about what Madeleine had been doing in the crèche. Who she was talking about is anyone's guess if Madeleine wasn't with her. It's quite mad IMO