What ways is the plot of The Scarlet Letter driven (almost mandated) by the motivations of its main character?

The novel is indeed driven by Hester's motivations. They account directly for the circumstances of the story and the events as they unfold.

First of all, the story could not have happened at all if, after Pearl's birth, Hester had chosen to leave the Puritan community. She could have taken Pearl (as she did seven years later) and returned to England. Instead, motivated by her deep love for Dimmesdale and her desire to be near him, she chose to remain and bear humiliation and punishment.

By keeping Chillingworth's real identity a secret, Hester enabled his quest for revenge. Once he realized he had found Pearl's father, Chillingworth then dedicated himself to torturing Dimmesdale, psychologically and perhaps even physically. (There is a suggestion Chillingworth was poisoning Dimmesdale.) The old physician's relationship with Dimmesdale and the effects of his revenge upon Dimmesdale comprise most of the story, all brought about by Hester's agreeing to meet Chillingworth's demand. Why did Hester agree to never acknowledge Chillingworth as her husband? She was motivated by guilt for how she had betrayed him; she knew the shame and humiliation her act brought to him. Also, Hester keeps Chillingworth's secret to protect Arthur Dimmesdale. Chillingworth threatened he would surely find and harm the father of her child if she revealed him for who he was.

Hester finally reveals Chillingworth's identity to Arthur because she is motivated to save his life and to convince him to leave the settlement with her and Pearl. Hester has watched Dimmesdale waste away; she is aware of his suffering. Meeting him in the forest, she tells him the truth. As a result, they both profess their love and plan to run away. This throws Arthur into his most severe and final test of moral principles and spiritual beliefs. Instead of leaving, he climbs the scaffold, confesses, feels forgiveness, and dies. Thus the main story ends, its major conflicts resolved.

All that remains is Hester's return to Boston many years later, where she lives out the remainder of her life, dies, and is buried next to Arthur. These events, too, result from her motivation. Hester's reasons for returning remain somewhat unclear, although the narrator suggests what they might have been.