I will add on to the answer above by exploring the symbolism and meaning behind Gerard Duval's death. During the time that Paul Baumer has spent fighting in the war, he has never killed someone up close. While in the shell hole, he plans what he will do if and when an opposing soldier falls into the hole. When Duval falls into the hole and Paul fatally wounds him, Paul begins to consider that the enemy soldiers are simply men just like him who are dying senseless deaths in the war. Duval's death is a symbol of Paul's complete loss of innocence--Paul realizes that he cannot undo the death which has occurred by his own hand. So Duval's death serves as a sign of the brutality that is the reality of what many consider a patriotic duty.
I will just pick out one portion of your question and answer it. THe symbolism of Kantorek is pretty straight forward and to me, also particularly relevant given the overall message and theme of the book as an anti-war novel.
His name itself is likely derived from that of a "cantor" or the person chiefly responsible for leading the singing in a church service, particularly a Catholic one. Kantorek leads the chants of the boys, to an extent, prior to the war as he tries to instill in them the image of war as glorious and a place where boys become men, etc. He leaves out the horror and the ragged madness of the battlefield, largely because he's had no real experience himself.
This is emphasized again when he is shown as a particularly ineffective soldier.