How to drive up steep mile-long hill?

Good response, but not 100% technically accurate. MG1 has an RPM limit which limits how fast the ICE can spin at low vehicle speeds. The ICE can't reach it's torque peak at 4000 RPM until about 40 MPH, and doesn't reach it's power peak until about 60 mph (not sure about this speed, and suspect it varies based on the battery state of charge).

At low speeds, it's all about the torque provided by MG2, which is about 400 lb ft. MG2 will maintain that torque level as speed increases until it reaches its 60 kW power limit, at which point further speed increases will see torque reductions in line with the motor's power limit.