When you speak of chapters 21-30, you are speaking of the latter parts of the novel The Last Crossing by Guy Vanderhaeghe. You are right in suggesting that there are important parts to the plot revealed here (especially in regards to the rising action, climax, etc.). It is important to review the elements of plot first, and then to explore the answer to your question.
Plot, of course, consists of five important elements: exposition (where we learn about the characters and the setting), inciting incident / conflict (where a problem arises causing issues for the characters), rising action (where the tension rises), climax (the highest point of the tension), falling action (as tension begins to fall), and resolution/denouement (where things return basically to normal). Let’s review these elements in light of your second question about the events significant to the plot of the entire novel, The Last Crossing by Guy Vanderhaeghe.
The plot of The Last Crossing is fairly straightforward. In the earlier parts of the book we learn about the two wealthy, English brothers: Charles and Addington Gaunt as well as their father and their missing brother (Simon) and eventually their guide (Potts) and the love interests (Lucy and Madge) and the setting of time (the 19th century) and place (England and the American West). The inciting incident (that some people call the conflict) introduces the problem of Simon, the brother who has become missing in the American West of Montana.
There are a great number of events that happen in the rising action (some of which happens with chapters 21-30). Earlier on, we learn more about a guide that the two brothers hire (Jerry Potts) and, interestingly enough, a journalist they hire as well (Caleb Ayto). In some way, during the rising action, each character has different issues from their past that is brought up to the reader. Everyone seems to have either a skeleton in the closet or a personal demon. The tension really climbs when Lucy’s sister, Madge, is both raped and murdered. The climax, in my opinion (although is the subject of much controversy), is when Madge’s murderer is revealed. It is at this point when the tension begins to fall and things get back to normal in the old west.
In conclusion, it’s important to note that a large part of the novel is about the culture clash of the European settlers and the Native Americans (especially in regards to Potts and his character issues). Smallpox referred to as “white scabs” and other interesting issues are important here.
The whites are proud of their blood, always boasting that theirs is stronger than the blood of any other people.
This becomes a significant theme in the midst of the novel. As a result, this novel becomes more of a character study of the various and sundry caricatures present in the Old West.