How was Léonard able to magically find an abandoned mansion in which to hold Raoul hostage? His police file reveals that he's from Rouen, a city in Normandy that is fairly close to Étretat—which is to say, he probably knows the area well.
The elderly woman who gets scammed by Assane in the 2006 flashback derived most of her wealth from plundered treasures from the then-Belgian Congo. Accordingly, she has a Flemish surname (Van der Meulen).
Assane's fixation on Arsène Lupin really isn't that different from users of sites such as Tumblr and Twitter "kinning" characters—he just takes things much further.
Assane's installation of security cameras around Dumont's house in order to keep track of his daily plans is presented as being yet another of his ingenious plots. But it becomes a lot more creepy when you realize that he has the ability to see the entire Dumont family eating, showering, sleeping and doing all manner of other things.
Because Assane had withheld critical information about his revenge quest, meaning that Claire didn't have a strong reason to immediately suspect foul play, it's likely that her greatest fear when she couldn't find Raoul on the beach at Étretat was that he had somehow drowned in the ocean.
There are indications that Pascal was instructed to do a bit more than just arrest Assane after Claire agreed to give him up in exchange for Raoul. Notably, he orders Claire to let Raoul into the apartment and then go straight to her bedroom, and is shown reaching into a suspicious-looking satchel. It's possible that this contained a gun, which would suggest that he was planning to shoot Assane—when his ex-wife and child were right there.
Although Raoul does wind up being correct that Assane is faking his death, his initial reaction comes across more like extreme denial than anything else. Claire certainly believes this to be the case, but feels powerless in getting Raoul to open up to her about his grief. She becomes so worried about his mental health that upon encountering his basketball coach for the first time, she immediately gives the guy her phone number—despite the fact that (in her understanding, at least) they've never met before—and implores him to let her know if Raoul is struggling to cope.
