Explain the symbolism of Kemmerich's and Duval's death.

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.