Consider starting the essay by explaining the context of the Elihu Lockton quote. He is addressing Bellingham, who is investigating the accusation that Lockton is on the side of England.
Bellingham asks Lockton if he has come to fight those who “strive for freedom and liberty.”
Lockton quips, “Freedom and liberty has many meanings.” He then asks if he’s “free” to go home. He wonders if he should be “at liberty” from the committee’s investigation.
An essay might note the irony of Lockton’s predicament. While Lockton complains that Bellingham is infringing upon his freedom and liberty (with quite a bit of justification; he is on the side of the British), Isabel could complain that Lockton and his wife are infringing upon her and her sister's own freedom and liberty (without any justification; they should be free). From Isabel’s point of view, Lockton would probably come off as a hypocrite. Sure, he cares about freedom and liberty when it pertains to him, but not freedom and liberty as it relates to Isabel and her sister.
An essay might also note how freedom and liberty is not linked to one country or ideology for Isabel. For Isabel, freedom and liberty is synonymous with individual survival. If she has to be on the side of the British to gain freedom, she'll try that. If she has to be a spy for America to achieve freedom, she’ll do that too.
Isabel’s shifting allegiances undercut Lockton’s and Bellingham’s idea that one country has a monopoly over freedom and liberty. If America is on the side of freedom and liberty, it shouldn’t be enslaving millions upon millions of people. If England is on the side of liberty and freedom, then its champions shouldn’t be keeping slaves either.
In a sense, Isabel represents the personal, specific complexities of freedom and liberty. She shows that these words truly mean different things to different people—especially among people with different skin colors.
Finally, an essay on Isabel’s point of view on freedom and liberty might note how freedom and liberty comes with obligations. Once Isabel can pass herself off as a free, she doesn’t abandon the people she cares about. She recuses Curzon from jail and then she goes to find her little sister. Unlike Lockton, Isabel seems to believe that if someone has freedom and liberty, they should use it to try and give others freedom and liberty as well.