I've done various parts of your proposed route. There are not a lot of options for most of the route. I started at the Canadian border , north of Lake Pend Oreille, and cycled down to Southern Oregon. I just used Highway 95 to Couer d'Alene. I headed south from there to Lewiston, and then into WA-- not the way you are going.
You would probably head west from Couer d'Alene to Spokane. If you are going through Yakima to Mt. Rainirer your route is rather limited. Once you leave the park you can hit HWY 706 and connecting roads to head for Olympia. When we did the Pacific Coast route we came down the Hood Canal which took us pretty close to Olympia. Hwy 8 connects to the PCR and will take you into Aberdeen and HWY 101. There are not a lot of options. A good source of information about the Pacific Coast Route from Aberdeen south is the book, Bicycling the Pacific Coast. However, there is not much navigation required once you reach the coast. Hwy 101 south until you get to Hwy 1 in California. The ocean stays on your right

Good luck on your ride-especially for the part from Spokane to Yakima. It is hot, windy and exposed.