A sting in the tale

"After the crash" the first novel by Michel Bussi, has a wonderful premise -

On the night of 22 December 1980, a plane crashes on the Franco-Swiss border and is engulfed in flames. 168 out of 169 passengers are killed instantly. The miraculous sole survivor is a three-month-old baby girl. Two families, one rich, the other poor, step forward to claim her, sparking an investigation that will last for almost two decades. Is she Lyse-Rose or Emilie?
Eighteen years later, having failed to discover the truth, private detective Credule Grand-Duc plans to take his own life, but not before placing an account of his investigation in the girl's hands. But, as he sits at his desk about to pull the trigger, he uncovers a secret that changes everything - then is killed before he can breathe a word of it to anyone...