Yes, neutron moderator is what the water shield does. Water was chosen for the high hydrogen content and low cost, so reducing shielding weight will decrease it’s effectiveness. The ability to do this in good conscience will vary with each installation. In the case of mobile installations, I would think no reduction would be conscionable. In large aircraft installations, several hundred tons of fuel and fuel system are no longer needed, which helps immensely.
Point #2 I’m not able to comment on.
Regarding Point 3, the DPF can be designed with a single piece cathode, which could double as the vacuum chamber wall.
