Unsure what you mean by your first paragraph so my answer to your question must be “No”
CO detectors are mandatory for all solid fuel stove installations in UK and you should have one as a prudent precaution.
When you close off the air inlet there is virtually no draft up the chimney hence the very real danger of CO leakage unless you have a proven running exhaust fan. Your existing manufacturer’s control system should have a sensor device which verifies the exhaust fan is running and sets off an alarm if it is not. Building a DIY control device for something that could seriously endanger your life just to save some money might not be the wisest decision in the world.
You are correct, there is an encoder in the exhaust fan that is controlled by the microcontroller.
I’m pretty sure that the microcontroller or other components doesn’t “close” the air inlet. What it does to turn the stove off is to stop the motor of the auger feeding the pellets and turn the exhaust motor to max power to cool the stove down and suck out the CO. So yes, because the air inlet is not closed and air can circulate freely into the stove, there would be some draft up the chimney that it is usually at temps higher than 100ºC when operating. Anyway, with two CO detectors near it as a precaution should be fine.
But yes, cosidering everything, I think it’s too much effort and too many variables to make something as good as the commercial controller, so I might end up just getting a new one and that’s it…