If throttle down more 20s in air for fast descend, than copter will disarm or not in air

Yesterday I fly my copter at altitude 150m , than I was need to fast descend my copter at safe altitude, In this case , I dont throttle down at minimum because I was confuse copter should not disarm in air due to zero throttle, because at zero throttle copter disarm normally.
Please guide, suggest related all parameters so I will set them properly.
Also provide parameters related with time settings of zero throttle disarm time, zero throttle left rudder/yaw disarm time, stop zero throttle disarm feature and descend/climb speed.
Please guide

I’m by no means an expert because I’m learning too . . . but with regards to the first part of your question . . . I believe mine only goes down to 5% throttle minimum with the stick, and only goes to 0% if the dedicated throttle switch is throw to disable. I’m not aware of the 20sec time limit you refer to.
I am also under the impression that a heli would auto-rotate to land even if throttle was completely disabled in flight, assuming safe altitude?

If done in stabilise mode, there is no attitude control on lowest throttle, only mot_spin_idle.
You need to disable crash detection to do that, or do it in sport or acro mode.