In Shining Armor doesn't affect Power Armor. Power Armor Training just trains you to wear the suit.
Err yes it does, In Shining Armor's description is misleading but in fact says "metallic armor" so it effects armor that is metalic
Power Armor should get +5 DT or +25% DR. I would prefer the +25% DR because that would effectively put me in "walking tank" status. It would block the entire 20% damage of weapons that cant overwhelm your DT.
You can already get DR by taking slasher or Med-X but that doesn't negate damage of a weapon that failed to overcome your DT. It just reduce it even further.
Criticals would still ignore your DT and DR though and Light Armor wearers with "Light Touch" are now invulnerable to critical damage so why not?
Light Touch characters aren't immune to critical damage they just make a rare event a little bit rarer. Also criticals do not ignore DT or DR.