因为QFT用的是Heisenberg picture/representation，做QM和QFT的类比的话要熟悉QM的Heisenberg picture/representation
QFT用的CCR(canonical commutation relation)的表示是Fock rep.
你可以去简单的看一下Wightman axiomatic QFT，再比对一下QM的axiom，就很清楚了。

For 5, because wavefunctions are cumbersome for the purpose of calculation scattering amplitude, for N particles you*ll need at least 3N coordinates to write the states down. In fact some people did calculations this way in the early days of particles physics, and it was soon abandoned.