a) How does the ionization efficiency in EI depend on the energy of the ionizing electron?
The ionization efficiency is close to zero with electrons carrying just the IE of the neutral under study. However, a slight increase in electron energy brings about a steady increase in ionization efficiency.
b) Why is a primary electron energy of 70 eV the standard setting in EI mode?
Because ionization efficieny is close to optimum at 70 eV for almost any organic molecule. This also allows for easy comparison of EI spectra obtained from different instruments.