You can be as cynical as you like about IOSA audits and certainly no audit can guarantee safety but audit firms do need to ensure that they do a proper job since there may be the potential for them to be sued by plaintives if there is an air crash - could IATA also be sued in these circumstances? Another 'deep pocket' for the lawyers to go after!
Another comment. Why do you think everyone is spending all this time and money on audits by non-government organizations? It's because not all States have the ability or can be trusted to carry out proper over sight of their airlines.