Now I have answered your first question: (1) How to set trigger CO leakage alarm, using minimum effort with minimum additional components. This answer need no additional components. For your second question of measuring exacting values, you need an ADC (Analog to Digital Converter). I checked that MQ9's analog output voltage range is a bit narrow, about 0.5V to 1.5V. So using low resolution ADC such as MCP3008 is not suitable.

One tutorial I referred mentioned using ADS1115 ADC with 16 bit resolution, which I think is more suitable. You can google tutorials for both ADCs. Perhaps I can write a simple python program to do ADC1115 later. Cheers.