I've done a section of the beaches during spring from Rialto Beach northward, and it can be incredibly wet. We had a nice mix of sunshine and driving rain, so I would advise that being prepared to encounter rain is the number one priority. October can be a really nice month up here, of course, but you just never know. The book "Backpacking Washington" has a couple of different coastal routes and suggest itineraries for trips. You can find this book here:
I have tarptents and in excessive rain found it to be a bit of a challenge with two persons in the tent because someone will invariably roll up against the side of the tent and wet out their bags. I fidget when I sleep, and I move around. If you don't as much, it probably wouldn't be a problem.
One thought on tides: if you have to do some of the overland routes, be prepared to go slow. The brush was incredibly thick in some places, and it took us a very long time to proceed. I was glad I wasn't using some of the lighter weight (more fragile) gear at a few points, instead opting for a heavier, yet tougher, pack. This allowed me to be a bit rougher on the equipment, which was made necessary by the terrain. Of course, there are a lot of people on this site that would probably challenge this notion. And by comparison to them, I am inexperienced in the outdoor arts. So consider the source!
In this regard, I would also suggest tuning down your mileage goals. The terrain is spectacular, the tide pools extensive and interesting, and the boulders sometimes prodigious. All in all, we went considerably slower than we expected because we spent so little time on the beach, instead opting to explore the tide pools and rocks or overland when the tide was high.
I can't remember if it was required, but it was at least heavily suggested that we carry a bear cannister. The reason had less to do with bears than those pesky raccoons, which know that people have good stuff to eat.
We didn't use the shuttle, but we also didn't leave much in the car. Since it was outside of the normal tourist season, we weren't as worried. I, too, am curious by the shuttle option.
I would also suggest coming up with a backup plan in the event the coast is rainy. Forks generally gets 100+ inches of rain annually, but October tends to be a bit drier than most months. Here is a annual rain forecast. http://www.forks-web.com/rainfall/
Finally, I'd suggest having a backup plan for east of the Cascades in the event the coast is rainy. One suggestion that might be worth considering if the weather is relatively good east of the Cascades and it is after Oct 15th, is to hike the Enchantments just outside of Leavenworth. You don't need a permit after Oct. 15th (which are very hard to get) and the larch is changing color. Personally, I'd camp at Upper or Lower Snow Lakes at this time and dayhike up to the Enchantments, especially if you are in a tarptent. (it can get very windy up in the Enchantments and there isn't as much cover). You can find out more here:
Anyhow, have fun on your trip!