Throttle curve is a 5 point curve set with the following Params
H_RSC_THRCRV_0
H_RSC_THRCRV_25
H_RSC_THRCRV_50
H_RSC_THRCRV_75
H_RSC_THRCRV_100
You can find this on the heli page in QGC or MP in addition to the full parameter list in MP
This curve is based on the collective where H_RSC_THRCRV_0 Is the throttle setting for the H_COL_MIN and H_RSC_THRCRV_100 is the throttle setting for H_COL_MAX. So even though you have collective at the min value doesn’t mean that throttle should be zero. The throttle should be at a value that keeps the rotor spinning at the desired rotor speed. Then the rest of the curve is set to maintain a constant speed as the collective loads the rotor. The motor interlock (ch 8) is designed to turn on/off the motor. You will not need to set H_RSC_IDLE. That is used for combustion engines.
Check out this post. It may help explain the throttle curve and how it works.