The driving force in Westmark is Cabbarus, and the characters can be defined by their responses to his governance. While we don't actually see much of Cabbarus's activities, we are lead to believe he is acting in a tyrannical manner: restricting the free press, murdering his political enemies, and incarcerating innocents. (I think it isn't clear whether all his actions are directed towards assuming the throne, or merely reflect his theory of governance.) How can his ambitions and excesses be countered?
Queen Caroline's resistance is through personal connections and privileges. As queen, she can legally block his adoption as heir. As the King's wife she can try to blunt his influence. As a member of the court she can voice her disapproval of Cabbarus's actions. We see Queen Caroline in those who influence those with power to do right, who use power rightly, and who shame those who do wrong.
Doctor Torrens starts in the same camp as Queen Caroline; his position in court gives him personal influence, but more sharply limited. When it fails, he turns to notions of law and justice. Torrens seeks the help of "the people" to correct Cabbarus's actions, but he envisions a return to the status quo--- to the "legitimate monarchy" rather than Cabbarus's personal power. While others see the need for systematic change, Torrens focuses on the excesses and illegitimacy of Cabbarus's rule.
Theo's actions center around speaking truth to power. His first instinct is to naively turn himself in after the raid on his master's printing press. Later, as an essayist he describes the incident as honestly as he can. He assists others in making their pleas to the uncaring government. Theo is hesitant to act and uncomfortable with violence, but has little personal influence and little trust in the law. When confronted by Cabbarus, though, Theo is bold in accusing him of murder and worse. Keller's use of satire is also a truth-speaking response, although in a more subversive tone.
Florian and Justin instead seek revolution. Their aims are to overturn the old order rather than repairing it. While they seek justice, they feel no allegiance to law--- and this is what distinguishes them from Torrens, and to a great extent from Theo as well.
Finally, Las Bombas responds to abuse of power in a unique way--- by subversion. A criminal power is answered by a confidence artist adopting the trappings of power: education, military rank, and nobility. "Count" Las Bombas may be hounded by the law, but the corrupt nature of Cabbarus's government also gives him space within which to operate. If truth and honest work are punished, Las Bombas' puffery and fraud seem a rational response.
In Westmark these individual responses do not bear fruit, but there are plenty of other literary examples where they are more effective.
William de Worde (of Pratchett's The Truth) finds himself a lone voice for a truth that "matters". But he is more effective than Theo because he works within a city where law still holds sway, and at a crucial juncture where the truth matters. A week later, and de Worde's "right word in the right place" would be too late.
Moist von Lipwig (of Pratchett's Going Postal) is a successful subversive. A con artist like Las Bombas, he first schemes to turn his punishment towards his own enrichment. He defeats the wealth and power of the Grand Trunk through subverting the very integrity and speed of message that is their key strength. He finds comfort in the thought of "fooling" the city about his true nature for the rest of his life--- just knowing he could stop is enough. Moist has the advantage of knowing what he is; Los Bombas seems to alternate between "providing entertainment" and believing his own humbug. Los Bombas lacks direction, but the Patrician is present to make Moist a crooked peg in a crooked hole.
Pratchett's Samuel Vimes provides a better example of the power of law. Vimes knows that the watchmen need to be watched--- even himself. But Vimes' concept of law is expansive and progressive rather than reactionary. Vimes desires to go after the "big crimes", going as far as arresting two opposing armies, or the leader of the city. Where Torrens seeks restoration--- putting the law back in its proper place--- Vimes seeks to expand the reach of the law, to eliminate privilege and bring accountability. Vimes' success lies not just in his established role but also his concept of justice, which Torrens lacks.
Even Queen Caroline's approach has its time and place. In Sethra Lavode, Khaavren uses his personal connection to make a plea for justice for his friend Pel:
"I do not presume to teach my Empress how to behave. But I have been around the court, and on the field of battle, and in the dueling circle, often enough to recognize a great heart; and a great heart cannot be lied to. Your Majesty, my only wish is to serve you--- to somehow do some small thing to in part atone for my failures. How could I, then, live with myself if I permitted my friend to be dishonored, and in so doing, permitted my Empress to dishonor herself, when I might prevent it? Or, for that matter, even if I could not, when I could see the way clear to try? That a task is impossible is no excuse for not attempting it, not when my heart tells me it must be done.
Westmark's climax finds the end of Cabbarus's power in Mickle, who embodies all of the contradictory and complementary responses of the other characters. As the missing princess, she is uniquely positioned to use her person and privilege to thwart him in a way that Queen Caroline cannot. As heir to the throne, Princess Augusta embodies the restoration of "legitimate" rule that Torrens seeks. Her denunciation of Cabbarus completes the work of truth-telling that Theo started as an underground pamphleteer, and subverts Cabbarus' purpose in bringing her back to court. It is Mickle's background in thievery and fraud that puts her in a position to bring about justice.
As for revolution: Mickle seeks change (and maybe freedom), not personal power. Florian is unlikely to be satisfied with merely a coarser princess taking the throne--- but neither is Mickle a return to the status quo. She opens the door for further change.