Yes you can, the scent cartridges are optional and the unit will function normally as an air purifier without them.
No. Washing the filter will destroy it.
The red filter check LED will illuminate when it is time to replace the filter after 4 months of operation of the air purifier.
The scent check LED Light will turn on after 30 days of use. At this time you can replace the scent cartridge and reset the scent monitor. However, if you wish to replace the scent cartridge earlier, you may do so and follow the instructions for early reset as found on page 7 of the Owner's Manual.
In general it is recommended that you run your air purifier when you are at home to clean the air. The unit may be run 24/7 if preferred. This unit will be more effective when windows and doors are closed.
Ensure there is an Air Purifier Scent Cartridge inserted in the unit, and that the silver foil layer has been peeled of the back of it. Then try turning the Scent Control Knob to a higher level of release, farther to the right. Keep in mind that the higher the level of scent release and speed of the fan, these cartridges will need to be replaced sooner than 30 days.
The HEPA-Type filters in your unit were shipped in a plastic bag to ensure purity. Make sure this plastic bag has been removed before use. If you have been using the air purifier, the filters may be dirty. High amounts of contaminants can block the pores in the filter and stop the air from moving through it. Replace the filter(s) with the Febreze Dual Action replacement filter 'U'.