I have a Ej205 so this may not apply to you. When I cleaned my IACV I fiddled with the orientation of the stepper motor on the valve. Basically adjusting how much that valve is open at it's resting position. At first adjustment I had the same problem as you described. Rpm's would fall fast and would shut the car off.
Then I turned the motor ever so slightly so the valve is a bit more open in it's resting position and it cured my stalling issue. Rpms fall fast but the ECU or the Iacv "catches" it before it stalls.
Just offering this as something you can try. My Ecu tuning knowledge is nonexistent.
Edit: FYI I have a ACT 12lb fly and a light crank pulley