score:5
Rome2Rio suggests you fly to Santa Rosa, which is well north of the Bay Area, and almost halfway between San Francisco and Mendocino.
Alaska Airlines flies from SEA-STS, though for most flights you'll have a layover in PDX (I'd try to avoid this if possible). Direct flights are available, but apparently not every day of the week. And this is the only commercial airline serving that airport.
After that you should rent a car for the much shorter 100-mile drive. While there are buses, the one you would need runs only once a day and the one daily direct flight arrives after it leaves.