In relation to the issues you raise:
Motive - due to neglectful reason their daughter had an accident and die for which they could be blamed.
No car - Body could have been hidden or moved on foot
No body - people are convicted all the time without a body being found.
And the statistics show that parental / family / friends are by far the biggest perpatrators of such crimes.
But we already know that no evidence that Madeleine had had an accident was found. So that would remain the same with this simpler cover story.
I did say that in the simpler version the body could be hidden in the same unfindable place which they apparently took her too much earler. So no change there.
The statistics also show that stranger abduction does happen. Ask Ben Needham's mum.
The massive difference in the simple version of events is that it doesn't include shutters/windows, or Jane Tanner having to be coached into remembering what she apparently didn't witness including having to remember a description of an imaginary man carrying a child.
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They wouldn't have to talk their friends into covering for them and then spend the rest of their lives worrying in case someone let something slip - or had an attack of conscience.
They could dispose of the body when no-one was around - instead of at 10.00 oclock at night when all kinds of people/traffic could be expected - like the Smiths for example.
Madeleine would have been taken in the night after they got home - so no need to even discuss what happened with checks etc during the meal. They could even say that Madeleine had told them she had nasty dream the night before about a man being in her room.
Why on earth would they embark on such a messy concoction of a story involving so many terrible risks, complicated lies and other people when none of that was remotely necessary. That makes no sense to me whatsoever.