The Westing Game: Chapters 19 to 21 contain a great amount of new information. What information was the most significant?

You are correct. Chapters 19, 20, and 21 do contain a lot of new and important information for readers to take in. What information is most significant is up to individual readers, and those choices are likely to change depending on which read through you are on. I think that a key piece of information in chapter 19 is certainly finding out that Grace and Crow are related to each other and that they are related to the Windkloppels. As for chapter 20, readers and the judge realize that Sam Westing isn't likely to blame for Violet's death. Violet's mother is who pushed her to marry the senator, and Violet likely killed herself to avoid marrying him. In chapter 21, I think the most important piece of information occurs when Judge Ford tells Sandy about her almost beating Sam Westing at chess. She thought she had the win in hand, but then she realized that Westing gave up his queen on purpose and tricked her. It's not until you figure out that Sandy is Westing that this information is so awesome. Westing is a master game player and loves tricks. The entire story itself is about his game with these people, and the fact that he plays so many characters in the story shows that he is still just as capable of pulling off tricks to control a game.