Stupidity: As far as I can tell this is not a problem introduced by the novelization, but already coming from the original story: without Poirot's involvement, called upon by the murderer, the police would not have suspected murder at all, and still would have a witness with a rock solid alibi to talk about The Dream. If they really wanted another witness, they could and should have summoned a psychiatrist and do to them the same stage act they did with Poirot, they would have been much more qualified witnesses to frame it as suicide. Even to Poirot himself, it's the murderer who suggests the thought there could be foul play involved, at all! The plan makes zero sense because Poirot is not the ideal witness and they want to suggest the victim was mentally ill and suicidal, not that someone wanted to kill him.
Stupidity: Hard to swallow that a group of vastly experienced policemen smart enough to figure other tricks out (such as the abnormal position of the gun) would be in any doubt about the victim being left or right handed when she is still wearing their watch on the right wrist - and attention is called by Poirot upon that particular detail right from the start.
Answer: He definitely says "Belgian", but the subtitles get it wrong and show him saying "American."
Wblank71