My list of suspects would be:
Greg - revenge / Ollie really his son
Mark - conveniently placed to disrupt the investigation and we know he was there when Ollie went missing.
Romanian Gang leader - seen outside house where Ollie was held
Garage men - directed Ollie's family to Hotel L'Eden, could keep them in the area by taking their time fixing the car.
Sylvie & Alain - because of Alain's mysterious meeting after Ollie's family arrived at Hotel L'Eden. Possibly related to corrupt mayor.
Georges - mayor, so can control investigation, related to Alain (Hotel L'Eden) and friend of Ian, who we know is dodgy. Seems to do Ian favours.
Ian - known paedophile.
Vincent - associated with Ian, has a false alibi, in the area when Ollie disappeared.
Malik - don't like him. He says he "knows" what happened to Ollie. Would do anything for a story.
Mary - took Ollie to replace Molly because she's already a bit unbalanced? Maybe Ian covered it up?
Emily - scared that Ollie was not safe around Tony due to Tony's temper? Perhaps Tony did cause the fracture? But she does seem genuinely distraught so I don't think she's very likely.
Corrupt police officer
I don't suspect:
Tony - genuinely distraught and why would he still be searching for Ollie.
Baptiste - too lovely
Laurence - too lovely
Monique - no motive
Have I forgotten anyone?