Purposes and Motivations aren't really pertinent to the Objective Story's Thematic arenas. Rather than looking at what the Characters are doing, keep in mind that the Objective Throughline represents a point of view for the audience. From the objective view they will see not only characters, but plot, theme, and genre as well. Of course, this is most clearly seen in the Storyforming stage, and from encoding onward, the view may not be as consistent or clear.
So the point is, forget about characters when using this report and consider the whole point of view. Using the report this way means that the Act itself centers on an exploration of the Past. In other words, when you are exploring the grand scheme of the big picture of your story in an arm's distance sort of way that gives the audience a change to look at the dynamics involved without being personally involved, THEN you will be examining the Past, in Act 1.
Another way to say this is that all four throughlines will have an area around which they center in Act 1. The Past will be one of those four items that serve as the focus of attention for the audience. In your story, in Act 1, the Past will be looked at Objectively (or impersonally, though not necessarily without feeling.)
Now we add in the thematics. What kind of things about the Past will the audience be looking at? Or, turned around a bit, what measuring sticks will be used to judge things that happened in the Past? The answer is: Rationalization, Obligation, Commitment, and Responsibility. These four items describe more specifically than just the notion of "The Past" the areas of interest in the Past that Act 1 will explore most closely from an Objective point of view.
So, look at the wide-ranging plot events, the behaviors that affect or are exhibited by all your characters, the overall genre of your story as it develops in Act 1, and then see that from an Objective sense. Your audience will see these things as all revolving around the Past and being examined in terms of Rationalization, Obligation, Commitment, and Responsibility.