I think you should give it a shot
If you're going to be in Seattle for a few days, and probably not doing as much driving for a day or two, then why not go for a long but great day on the Richland-Seattle leg? Like I said, you could cut it short if it doesn't work out and then make daytrips to Rainier or Snoqualmie.
How are you going home? You could also hit Mt. St. Helens on the way home...if you're going home via I-5 South, that is. If you decide to wait until going home to see Mt. St. Helens, you could see Rainier and Snoqualmie Falls via this route. Then, on the way home, see Mt. St. Helens from the other side.
What are your plans for the Olympic Peninsula? I'd be glad to help you out with that area as well.
It's always nice to have more time
No matter how much time you have, more is always better. Ah, well....
No, you won't be able to see everything in Seattle worth seeing in one day. Not a chance. You will have to make some difficult choices.
Some of the best of the highlights might include starting early at the Farmer's/Pike Place Market (watch out for flying fish!), then down to the waterfront and a visit to the Ye Olde Curiousity Shoppe, then a walk down to Pioneer Square (where Seattle first started, the historic district) where you can take the infamous underground tour of Seattle (lots of fun and quite interesting) which start at Doc Maynard's, and then a walk back to downtown Seattle area to the Westlake Mall (about 10 long city blocks) to take the monorail (left over from the 1962 World's Fair) to Seattle Center. Take a ride up the Space Needle and, if you have time, maybe a quick tour of the EMP/Experience Music Project in the fascinating Gehry-designed building. If nothing else, walk around the outside this unique building.
This would give you a good taste of Seattle but there's a lot more to see. Lake Union and its houseboats, Hiram Chittenden Locks with the salmon ladders and arboretum/gardens, the very unique Fremont District, Queen Anne Hill and the Victorian mansions, Lake Washington, Burke-Gilman Trail, yada yada. But the stuff in the paragraph above is probably the best to start with.
Possibly your friend has other ideas I haven't mentioned that would be worth doing, too.
Oh, eat at one of the Ivar's locations, preferably the one on the waterfront which is the original location. Ivar is gone now but he was a Seattle institution, for many years. I can still sing the "At Ivar's...acres of clams" theme song. :)
You route would work. You'll miss the heart of the Oregon Coast but the California coast, particularly if you do the stretch on CA-1, will make up for it. Enjoy!