What are some major events in Chapters 18, 19, 20, and 21? What is the climax in these chapters? What could have Isabel done to avoid the situation...

The main focus of Chapter 18 is the hanging of Thomas Hickory, accused of plotting the assassination of George Washington.

Isabel attends the hanging, and meets Curzon there, the slave who has persuaded her to spy on her masters. She asks him when she and Ruth will be able to leave New York, but he brushes her aside, saying they will be free eventually.

In Chapter 19, Isabel is forced to go to an Anglican...

The main focus of Chapter 18 is the hanging of Thomas Hickory, accused of plotting the assassination of George Washington.

Isabel attends the hanging, and meets Curzon there, the slave who has persuaded her to spy on her masters. She asks him when she and Ruth will be able to leave New York, but he brushes her aside, saying they will be free eventually.

In Chapter 19, Isabel is forced to go to an Anglican church service, and she compares it (unfavorably) to the open, straight-forward Congregational church she used to attend. 

The British arrive in town during the service, and the Tory congregation packs up to run home. Ruth is frightened and refuses to move for some time, but Isabel plays it off. 

What Isabel is not able to play off is when Ruth dumps muddy water in the washing bin where Isabel had been doing the laundry. Madam Lockton sees her and knows something is wrong with Ruth. 

In Chapter 20, the town is preparing for a big battle between the redcoats and the rebels and excitement is high.

Meanwhile, Isabel returns from an errand to hear that Madam has made the slaves gingerbread and milk and given them the night off, after begin lectured on her treatment of them by the preacher's wife. Isabel is suspicious, but Becky encourages her to enjoy it while it lasts. 

In Chapter 21, Isabel wakes up to discover that Madam has drugged her with the milk.

While she was sleeping, and Becky had the night off, Madam sold Ruth to be a doctor's maid in the Caribbean. 

At the climax of this section of the novel, Isabel confronts Madam for what she's done. Afraid of Isabel's anger and refusal to obey an order, Madam breaks a painting over her head.

Isabel runs to the rebel camp, demanding help, with Madam hot on her trail. Colonel Regan insists that Isabel return with Madam. Isabel tried to escape, but it hit on the head. 

As to the question of what Isabel could have done to escape the tragedies of Chapter 21, it's hard to say whether or not she could do anything. Certainly she has misplaced her trust in Madam's kindness and the Colonel's promises. Still, she had misgivings about both already, and trusted in them because she felt she had little choice. I would argue that these events were unavoidable, due to the way slavery limited Isabel's options.