I think another big challenge, Tarkus, would be where to build the bypass on the south end of US 26 (future I-84!

) Ideally, you could route at least an expressway along Cornelius Pass Rd, as it appears to have a fairly wide ROW on it's eastern side, and most of the new development near Cornell is on the western side. There'd be some challenges upgrading existing crossings to to interchanges, but it could be done. The routing south of TV Highway would be tough...
North of US 26 is a little less developed, and in theory, it'd be possible to keep the freeway routed along Cornelius Pass, with a new alignment being needed to connect with US 30. Then, like you said, across to Hayden Island, and into Clark County.
I agree that keeping the freeway out of Multnomah County would be nice, and would avoid a lot of Tri Met's influence. Dare to dream, right?
The last challenge would be where to have it meet with existing Clark County freeways. If it's possible to extend WA 14 westward, you could piggyback the new freeway along 14 to meet I-5.