A simple "no math" answer:
The higher the inertia, the more it can "resist" anything that wants to change its velocity.
a. High blade inertia is good pilot intervention time ( the time between engine fail and collective down which army requirement is 2 second.
It usually takes the pilot a few seconds to realize things are going wrong. Call it the "WTF!" moment. A high inertial rotor system will see less decay in speed during those moments. A successful auto-rotation requires preserving that rotor RPM.
b. High kinetic energy is good for soft tough down after flare in autorotation.
So you made it past the "WTF", kept the rotor speed up, picked out your landing spot, and are in full auto-rotation. As you approach the ground you now are going to need to "flare" - increase collective and haul back on the cyclic. The higher inertia rotor system allows for a "sloppier" flare because the inputs (to use the rotors stored energy to arrest the descent) will have less effect on the RPM.
c. What is the relationship between them?
In both cases, the higher inertia system works in the pilots favor.
An extreme example of a high inertia rotor is the (never ending) Carter-Copter Gyro project. They have "weighted" tips to increase rotor inertia.
Older video, but jump takeoff demo'd around 40 sec mark. The rotor on an auto-gyro can be "pre-rotated" with some assist from the engine, but when he makes that jump, that is all inertia working in his favor.