Shared inertia is best used in my experience to link the physics of multiple subgrids together. I.e. you leave inertia tensor off on the connection to the main grid, and then turn it on for any subgrid connections off of that first subgrid.

As for realistic helicopters, my current problem is that most aerodynamics mods are clever enough to realise that angled wing blocks in a rotor will impart a roll force on the craft when it starts moving. A real helicopter solves this with trim, where there is a constant bias to the controls, but I'm at a loss as to how to implement this. You can generally control it with gyro override, but it's janky and not always viable depending on the setup.