(of a liquid) agitated vigorously; in a state of turbulence
The setting of this scene is important because 1) the sea is the escape route to Sweden and 2) the sea's churning waves parallel the emotional churning inside Annemarie as she runs through the woods to deliver the packet, with the fears that she might not get there on time and that her mission might be discovered by Nazi soldiers.
She could hear the churning sea beyond, and see the wash of daylight to the east, over Sweden.
In telling the story of "Little Red Riding Hood" to Kirsti, Annemarie doesn't harass, criticize or carp (Kirsti is the one who carps with trivial objections). "Tantalize" means "to tease or torment by presenting something desirable to the view but continually keeping it out of reach"--this is what Annemarie does to Kirsti (Kirsti knows the story), and what the author Lowry is doing to her readers by prolonging this part through the woods.
Annemarie always tried to prolong this part, to build up the suspense and tantalize her sister.
wreaking or capable of wreaking complete destruction
"Withering" is used as an adjective here, but it is closer to the definitions of the verb "wither" ("shrink" or "lose freshness, vigor, or vitality"). Faced with soldiers, Annemarie is neither capable of nor wishing to wreak complete destruction; rather, she is trying to act like an angry Kirsti with a look meant to shame and push the soldiers back a little, so that they would not sense her fear and look under the napkin.
"Resistance" also means "the action of opposing something that you disagree with"--both definitions need to be combined for a more accurate description of the Danish Resistance, who were organized to oppose the Nazis not simply because they were abusing their power and occupying their country, but also because the Nazis were committing crimes against the Jews that the Danes did not agree with.
Peter, and others in the Resistance who work with him, bring them to me, and to the other fishermen as well. There are people who hide them and help them, along the way to Gilleleje.
But Annemarie forced herself to think of her redheaded almost-brother, and how devastating the day was when they received the news that Peter had been captured and executed by the Germans in the public square at Ryvangen, in Copenhagen.
"Bleak" also means "unpleasantly cold and damp," which along with the chosen definition, could be a fitting description of the physical ground in which the Nazis buried the bodies of the young men they had executed. "Bleak" ("offering little or no hope") could also describe the effect that all the numbered and unnamed graves, especially Peter's, has on Annemarie and her parents.
Later, Annemarie had gone to the place with her parents and they had laid flowers there, on the bleak, numbered ground.