HI Jaime,
Good ideas here. I've done Olympic coast trips on many occasions, from August to a great 4 day trip in January a couple years back. On the winter trip we expected really cold weather and hefty winds and it delivered, but the trip was GREAT! I've done the Hoh before too- another fantastic trip, as is Enchanted Valley and basically anything else in the Olympic NP.
On almost all of my trips out there I've used a Tarptent. Usually partners have them too. On the deep winter trip I was field testing a Rab Summit Extreme which was massive overkill. The other couple had a Tarptent Cloudburst which was just fine…I think my Squall would have worked just fine.
With the exception of Cape Alava, you typically have spots with at least some protection. And October seems very mild in my book for a Tarptent on the coast. I've never carried a bivy with a Tartptent and I doubt I ever will. When the weather is wacky, I just pitch them closer to the ground, put the packs in plastic, and use them to block any splash from heavy rain. Heck, I often use Tarptents above the treeline in hefty winds and have even had them in good snowfall with minimal issues. I think your Tarptent has you covered.
I'd trade the weight of the bivy for a lightweight umbrella if you do any forest approaches, like those from Lake Ozette. The Olympics are my #1 spot for an umbrella because the rain falls heavy and the temps are mild. I also typically use a pack liner AND a pack cover…especially when I'm using down.
I love the trip from Rialto to Cape Alava but yes, you do need to work a shuttle. Two cars are nice but the rangers can help you find a shuttle- I've used a "taxi" service from Forks before, although it was really just a nice guy and his car. :-)
Another sweet one is doing the triangle from Lake Ozette. From there you can go north (with a good ford) or south and explore some beautiful country. This is a wonderful place and in October, it should be faily lonely. Too many people, head away from the crowds to the north or south to total lonliness.
The Hoh is brilliant too, but a very different hike.
If you do the Ozette trip, you do need a bear canister, but mostly for racoons. They allow those plastic buckets and you can sometimes get them at the ranger station. When I've met the rangers, I explained and showed my Ursack and they were cool with that. The last few trips we've just used a bear bag and it's been fine (although I risked having to carry a huge plastic bucket…it's not oficially okay but my gut says that it's at least as effective as the buckets).
Last, this is a great place for poles. Walking sand, crossing slippery rocks, walking wet boardwalks…these are all ideal places for poles.
Oh- one more thing. There is a really cool side trip off of the Ozette loop where you can see an old pioneer cabin- the Roose Cabin. Shh…it's a secret.
No matter what you choose, have a GREAT trip! You are about to experience one of the truly incredible places in the lower 48!
Best wishes,
Doug