Why did the mother get in a grave?

Because she had given up on life. I read it as her pleading with her husband to fill up the grave with her in it so she could be with her dead sons and out of their miserable existence.

I took it as one last hug before Caleb's eternal dirt nap. Also for anyone who has ever dug a big hole that one was done by an engine driven backhoe. Probably borrowed from one of the unseen neighbors in the "village."

He killed sixteen Czechoslovakians. Guy was an interior decorator.

Because Caleb was precious in the mother's eyes or whatever. He was the second child of the family and the oldest brother anyway.

If you people would spend more time paying attention to what goes on the movie then maybe you wouldnt be nitpicking at this kind of stuff