Although Grisham writes fiction, he does not write idealised situations or characters. In aiming for a "realistic" portrayal, he may have otherwise decent characters do amoral or unpleasant things, or find themselves in situations where there is not a stock happy ending (I can't say for sure; I've never read Grisham). This is what I understand by "grim realism" - storytelling which is not sanitised for the readers' sake.

It is the noun "realism" modified by the adjective "grim". Everyone around the place place is grim. Is the Schroeder's cat experiment?

A general title for a few forms of art, like I read it in filmology.

In this case "grim realism" means something very serious. The author is stating that the responses are inadequate given the seriousness of the situation.