What an interesting idea for an assignment (which I assume will eventually be an essay). I find it interesting because the theme of confinement is not necessarily a common theme mentioned in Pale Horse, Pale Rider, but you can definitely make a case that the theme is both present in and important to the story.
First, let me comment on your original ideas. Your ideas are superb, but they just need to be honed a bit. You do too much summarizing in the beginning of your thought process here. However, your ideas behind the confinement theme are wonderful. For example, you are correct in asserting that Miranda is confined by influenza during the epidemic. Secondly, you can definitely prove that Miranda is further confined to live in a world without Adam after Adam dies. (Luckily, her friends will not accept Miranda giving up on life like this.) Your ideas about confinement, though, are hidden within sentences full of summary. If your goal is to discuss (and prove) the theme of confinement, then perhaps you should begin with an outline of how your essay will look.
Your introduction could contain many sentences of summary that you originally suggest. Then simply filter down to the theme of confinement and your thesis. A good thesis might be as follows: The theme of confinement can be found in Pale Horse, Pale Rider through Miranda’s original dream, Miranda’s job as a reporter, Miranda’s experience with influenza, and Miranda’s loss of Adam. Each of your body paragraphs should focus on each of these confining elements. Although you thoroughly explain confinement through the flu and through the death of Adam, I would suggest two new ideas that can be turned into body paragraphs: the confinement of Miranda’s original dream (about death) and the confinement of Miranda’s job as a reporter.
In regard to Miranda’s original confining dream, Miranda is pursued by death, who is the “stranger” in the dream, ready to ride with her at dawn on horses used for fox hunting. Miranda evades death in the dream by not riding with the “stranger” at dawn, but she admits there will be another time. How is this dream confining? It confines Miranda’s future thoughts to the subject of death and proves that her subconscious already has death as a focus.
Miranda’s job as a reporter is also confining (and could serve as another body paragraph). Miranda’s job as a reporter does help her earn a living but also causes her great stress. Miranda is pigeonholed into reviewing dramatic productions for the local newspaper. This involves continual confrontations with actors that Miranda has previously criticized through her writing. This is also confining in that Miranda is forced to attend both plays and vaudeville shows for her work instead of for pleasure.
Your conclusion should reword your thesis and then present a new idea. I would suggest that Adam as well as Miranda’s friends are the characters that help release her from confinement. Before Adam passes away, he mentally frees her from the stress of her job. After Adam passes away, it is her friends that will not allow her to be “confined” to the earth by sinking into depression. This is an interesting new concept to present in the conclusion of your essay.