I live in Moses Lake. I have accessed the Camino from both Paris and Madrid.
If your question is, "What is the quickest way to get from SeaTac to SJPP?", then the answer is fly Delta, non-stop, from SeaTac to Charles De Gaul (it leaves around 2 pm and arrives around 8 am). Once at Charles de Gaul, you take either the bus, a taxi, or the Metro (I prefer a taxi) to the Gare Montparnasse train station in the heart of the city. You then board a direct high speed train from Gare Montparnasse to Bayonne. At Bayonne, you either take the local train (which will be waiting for your train from Paris) or a taxi (which I prefer) to SJPP. If you use taxis in Paris and Bayonne, the trip takes about 19 hours (four of which you can nap on the high speed train).
Others will suggest flying to Paris and then take a flight from Paris to Biarritz before making your way to Bayonne by bus or taxi, and then onto SJPP. The problem with this route is that you may have to leave Charles de Gaul airport and travel to Orly airport to catch your flight. Or if you find a flight from Charles de Gaul to Biarritz, it may not leave for hours. Another problem is your arrival in Bayonne from Biarritz. The local train, which makes infrequent trips to SJPP will probably not be waiting.
So, while flying on a plane at 600 mph from Paris to southwest France may seem quicker than a 200 mph train, it's not when you consider the other issues. For me, sitting in Charles de Gaul for four hours while others are arriving in Bayonne on the train makes no sense. Especially if you are jet lagged. A four hour nap on the train beats sitting on your luggage at the airport.
Regarding your high speed train options, some might tell you to fly to Charles de Gaul and then catch a high speed train directly from the high speed train station at the airport to Bayonne. But there are two problems with this option. First, the trains from Charles de Gaul to Bayonne do not leave as frequently as the trains from Gare Montparnasse for Bayonne. So again, you are sitting at the airport--jet lagged--doing nothing while others making their way to Gare Montparnasse and are already catching a four hour nap.
The other problem with catching a train at Charles de Gaul is that those trains do not go directly to Bayonne. You will have to disembark and switch trains at some time. That becomes an issue because that requires you to remain awake and vigilant for your stop. That's the equivalent of tourist water-boarding. Again, if you board a direct train from Gare Montparnasse to Bayonne, you can set the alarm on your phone and grab several hours of sleep knowing that you do not have to worry about switching trains.
Now, there are those who would recommend flying into Madrid from Seattle. This has a whole host of problems.
First, there are no nonstop flights from SeaTac to Madrid. Your nonstop flights to Europe from SeaTac are: Paris (on Delta), London (on Delta and British Airways), Amsterdam (on Delta), Frankfurt (on Lufthansa and Condor), Munich (seasonally on Condor), and Reykjavik (on Icelander Air). This means a plane change to fly from Seattle to Madrid. Which costs you time. Some connections are tight (like less than an hour). This can cause a huge cascade of obvious problems. So to counteract this, you book a longer connection (if one is available). Which means you are once again sitting in an airport while others are in Paris, boarding a high speed train to Bayonne.
The second problem with making your way from Madrid to SJPP is that it takes longer than making your way from Paris to SJPP. The trip from Madrid to SJPP involves several combinations of trains, busses, and/or taxis. At best, you land in Madrid and make your way to Pamplona by bus or train. Then you can take milk run bus to SJPP. But, he last time I looked, the bus from Pamplona to SJPP leaves only once a day just before noon. Which means you have to spend the night in Pamplona. Not exactly a quick route. And more costly given the overnight in Pamplona.
The last issue of flying into Paris instead of Madrid is a romantic notion. If you fly into Paris, your first steps in Spain will be on a scenic dirt path high up in the Pyrenees instead of a concrete sidewalk outside the Madrid airport. Ever walked into a country? In the mountains? It's kind of cool.
Another suggestion you might receive about flying from Seattle to Madrid is to change planes in the United States. This is not a good idea for several reasons.
First, as previously discussed, getting from Madrid to SJPP from is more time consuming (and frankly, more of a hassle) than getting from Paris to SJPP.
Second, changing planes in the United States means that you are literally not flying in a straight line to Europe. Remember, most of Europe is above the Canadian/United States border. By flying on Delta from Seattle to Atlanta you are literally flying away from Europe at a 45 degree angle. A straight line from Seattle to Paris goes over northern Canada, Greenland, and then Iceland. I know that sounds crazy, but if you look at a globe (or Google Earth) you will see a straight line from Seattle to Paris goes over northern Canada, Greenland, and Iceland (remember, flat maps distort the true route that planes take). So, by flying to Atlanta, you are literally adding hours to your trip.
Now, if your question is, what is the cheapest way to get to SJPP from Seattle?, then all bets are off.
You look for the lowest airfare from Seattle to either Paris or Madrid, regardless of layovers. The cost of the train from Paris to SJPP is roughly the same cost as the train from Madrid to Pamplona. The only major issue is spending the extra day in Pamplona to catch the morning bus to SJPP.
But be forewarned, once you've been awake for 32 hours, and are in desperate need of sleep, saving $200 on a cheaper airfare no longer seems like a brilliant idea.
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From any of these gateways, take a short flight to Biarrittz, or take the train from Paris, and then an easy local train or taxi service like Express Bourricot transit to SJPP from there.
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