Norman states that the "last murders" occurred "four years ago," referring to the events of Psycho III. That film, like Psycho II, was set in 1982. This film was both made and set in 1990, meaning that Norman should have said that the last murders occurred eight years earlier.
When Norman poisons his mother and her lover they die in the basement. In Psycho, we are told that they are found dead in bed. In addition, the container holding the tea is differ from the old one we see in Psycho II.
Norman uses strychnine to poison his mother and her lover. The lethal dose of strychnine is around 10mg and Norman's bottle reads 1.5mg/oz. He puts a maximum of 4oz into the ice-tea. Even if his mother and her lover drank the whole pitcher, it would have hardly killed them. Furthermore, strychnine is one of the most bitter substances known to man, it's highly unlikely to be swallowed by accident, even in very small doses.
Young Norman Bates takes his dead mother out of the casket, hides the corpse in her room (to start the saga) and puts books instead as a weight. Yet in one of the previous Psycho movies, when years later police exhume his mother, they find a corpse in her casket and no books. However, it is presumable that, after discovering what Norman had done in the original film, they dug up the casket and put her dead body into the casket.