When the family are in Mr Fullerton's office, when we see a close shot of the family, Edmund is on the left, then, when we see a slightly wider shot he is on the right, and then back again.
Although common now, the business of carving pumpkins for hallowe'en is a relatively recent import from the USA. Even in the 60s (when the book is set) it was pretty much unknown in the UK. What people carved back then were swedes - it was an awful lot harder.
The green witch makeup and decorations at the party, while appropriate if set in the 1960s (as was the book), are out of place for a story set in the 1930s. Green was not a color associated with witches until Margaret Hamilton made the look famous as the Wicked Witch of the West in The Wizard of Oz (1939), where the green makeup was designed to take advantage of the Technicolor look of the movie.
So the old lady really did leave the property and her money to Olga and not to the old lady's family. Poirot found the original will. Therefore the inheritance should go to Olga's parents. No mention of such.
Poirot quotes Shakespeare, saying that "Methinks the lady doth protest too much". This is a common but an incorrect quote. The writing actually says: "The lady doth protest too much, methinks.". The word 'protest' doesn't mean denying anything, but the same as 'vowing' in the current English language.