I can take your main question.
The scene in Dr. Baker's office offers the book's second great plot twist, and serves as the novel's denouement. Both the heroine and the reader assume that Dr. Baker will reveal what Rebecca told Maxim that fateful night--that she was pregnant with Favell's child. But now we learn that Rebecca's statement to Maxim was merely another one of her many lies: indeed, perhaps this last deceit was even calculated to make Maxim kill her, and thereby to bring about his death when the murder was found out. Lastly, the fact that Rebecca was sterile resonates somewhat symbolically: just as no good can come of evil, so too can no child issue from Rebecca's womb. the mansion was Rebecca's home, and it is hard to imagine them living happily in a place still so haunted by her memory. As they drive along, the heroine dreams that she sits in the Manderley morning room, sending out invitations. But the cards are written in Rebecca's hand, not her own, and when she looks in the mirror, she sees Rebecca's face. She narrates, "And I saw then that she was sitting on a chair before the dressing-table in her bedroom, and Maxim was brushing her hair. He held her hair in his hands, and as he brushed it he wound it slowly into a long thick rope. It twisted like a snake, and he took hold of it with both hands and smiled at Rebecca and put it around his neck." The dream has a clear meaning: Rebecca's ghost is still strong; if the couple were to stay in Manderley, they would only be opening themselves up to her malignant force. The destruction of the mansion is a difficult burden to bear, but it frees them, once and for all, from the past.