a few speculations

Clues that i find interesting when reading:

1 Never explains how Jackie came to know about the change of the Doyles' itinerary, which is suspicious as Author seldom left details unexplained. and this suggests that Jackie might have an accomplice.

2.Never explains what Simon did in the interval between the time Cornelia and Ferguson leaving with Jackie and the time Ferguson returning for Simon, in which time there was some time for Simon to be totally alone in the salon. Unusual to be left unexplained, too.

3.The sun-moon metaphor. When the sun comes out, one cannot see the moon. However, the moon is always there. When the sun goes down, the moon is there again. There must be something that this metaphor came out so early in the book and later proved to be true as a prophecy. Unlikely just to be a coincidence.

4. Poirot was particularly fond of Jackie. Great minds attract each other. So could it be that one is for crimes and another for solving them?

4. Jackie had the most obvious motive, and Simon had probably the best motive; but both had the most valid alibi. Come to think of Author's habit, the most valid alibi combined with the most heartbreaking motive, you've got a fascinating crime.