What do the actions of Mrs.Whatsit and Co. tell you about the extent of their power to defeat evil?

This is pretty much a thematic question. The novel's overarching theme is characterized as a battle between the forces of good and the forces of evil. L'Engle's characters all generally line up in one of these two categories and there is very little overlap or moral ambiguity between the two. Characters such as Mrs. Whatsit, Mrs. Which and Mrs. Who represent good in the world, while the forces of evil are represented metaphysically by the Dark Thing or the Black Thing, and symbolized by characters such as the man with the red eyes and IT. Meg Murry and her family inhabit the space in which good and evil battle over dominion in the world.

In the novel, good and the characters represented by good take on the traits of freedom, equality and creativity, while the characters of evil inhabit the qualities of hatred and conformity. Meg's battle, thus, is between submitting to the forces of evil or embracing the light of goodness.