How connected are Pierce and Yakima though? IIRC there's a pass there but it probably makes more sense to cross the Cascades in the North or South.
Actually indeed it turns out they are not connected by road (there is a pass but the roads force you into other counties shortly after it anyways).
That can easily be solved by crossing the Cascades further south on Lewis County instead of Pierce (so the 4th takes the eastern 2/3 of Lewis County and the 10th takes the portion in Pierce), and has no real effects on partisanship.
Chinook and Cayuse passes are closed in Winter (and are still closed for 2020) so a connection between Pierce and Yakima is out (no greater COI can exist if the areas can not even communicate in an ordinary manner).
That a highway route passes through other counties briefly is not significant in my mind. If you think of the two counties as two blocks of wood glued together, with the highway as an iron rod placed to restrict torsional pressure on the joint, then it doesn't matter if the rod is partially external.
I would allow use of Stevens Pass between Snohomish and Chelan; Snoqualmie Pass between King and Kittitas; White Pass between Lewis and Pierce; and the Columbia River between Clark and Klickitat.
Historically, the Columbia River was the route to the eastern part of the state. When Washington was a portion of Oregon Territory, the territorial legislature created Lewis and Clarke (sic) counties. Clarke went upstream from Fort Vancouver, and included all of the Columbia Basin east of the Cascades. Lewis was the area west of the Cascades.
Modern Clark is the remainder of the original Clarke. Modern Lewis is what was left after they chopped off additional counties. In particular, Thurston included the entire Puget Sound areas.
