Air Purifier Circuit Mod Help
It seems you want to defeat the auto control method and have some personal need to control it remotely and defeat all the uC functions for the sensor and switches.  This implies a steady state operation and can be monitored on the connector CN1.  If you measure the voltages on each pin for each level (after you locate the 0V reference pin on the PIC processor. ) 

Then you can toss the board and just use jumpers to make connections on CN1 to enable the fan at full speed and ionizer all the time.

But I am not sure of the value of doing this unless the noise of the fan keeps you awake at night.  Then I would look at a better fan or a better quality air filter.