Plot hole: Basil torments Mrs. Richards by miming when he is speaking to her, so she will think her hearing aid has failed. However, she speaks twice during the scene, and she would hear herself - obviously - and would know that he was trying to fool her.
Plot hole: Sybil leaves the hotel at 05:29 and returns with Audrey at 24:39. 19 minutes have elapsed in real time during which Basil has been 'entertaining' Sybil's friends. So Sybil had just enough time to visit Audrey, talk to her and return. There wasn't enough time to visit the homes of Alice, Kitty or Virginia. Sybil went straight to Audrey who must therefore be her closest friend. Also Audrey was free and knew nothing about the party. Would not Basil have invited Audrey to the party? Is not Audrey's presence just to allow Sybil to explain to the audience when it makes no sense she is not at the party?