I remember the case, but have not seen the documentary.
The French police may have made mistakes, or may not, I could not tell.
I have noticed over the years that when a crime occurs involving any nation's citizens who are not in their own country when murdered, raped, etc. etc. people always automatically criticise the police investigation,
Our police would never have done that is plastered all over the reporting of the case, and no one ever stops to think that "our police" may very well have done the same, and been criticised for doing it, when a crime occured on their own turf.
This particular crime did sound more like a spy novel than most spy novels do, and although we all know by now that psycopaths exist and may kill people at random, it is not usually the most reasonable explanation.
Unless new evidence turns up, or someone confesses to the crime on their death-bed, and does so convincingly, I don't believe this crime or many others like it, will ever be solved.
I may have missed an announcement, but although the German police said they had found Madeleine McCann's murderer, I think I am right in saying that no-one has yet found the child's body, or convincingly explained why a pedarast known for liking little boys should have abducted a little girl. Neither has anyone yet told us who actually killed de Lorca and where his body is, although practically each and every group that took part in the Spanish Civil War has been accused of his murder.