On Whidbey Island you can detour off of SR 520 a little bit north of Freeland and use Bush Point/Smugglers Cove Road. Gets you away from the traffic for a bit, and it's not too hilly. South Whidbey Island State Park is on this detour, so it's a good camping option.
Yeah I definitely agree with this as well, I ran out of time updating that map and missed that section. Might add a couple more miles but time off of 520 is worth it. I grew up on Whidbey and Fidalgo Islands and love riding there. Lots of scenic areas and nice rides with some pretty good hills for sure.
Regarding the two options on north Whidbey, I've done both. I would take the "detour Oak Harbor" option the next time. While it is much hillier and a tad longer, the traffic is minimal and the views great. The through Oak Harbor route is shorter and flatter, but Oak Harbor drivers...eh. This was the only place where I got honked/yelled at from my last tour between Portland and Vancouver BC.
Yeah I always go out to West Beach Road - there are a couple of tough hills (worse going North to South, but still worth it) but the scenery is fantastic and Oak Harbor is a sh*t hole (and I say that as a former resident!). Considering the hills and a fully loaded bicycle I think the 100-110 miles from SeaTac to Anacortes is pretty tough. Stopping at South Whidbey State Park is a great option but if one wanted a few more miles camping at Deception Pass State Park is another great place. That puts you right on the edge of Fidalgo Island and you could easily start the Northern Tier from that point (it'd only add maybe 15 miles).
So here's an updated map with my absolute preferred route on the islands, which is about 110 miles: http://g.co/maps/hw2up
If you stop at Deception Pass it's right about 100 http://g.co/maps/uaj5a
Stopping at South Whidbey State Park it is 62: http://g.co/maps/pq7xk