Longtime Seattle resident and European traveler here with a few thoughts to contribute...
1 - Nonstop flights are great, maybe wonderful, but don't obsess too much over them. And I say that as someone who is VERY picky about my flights.
Good news: There are more intercontinental nonstops from Seatac now than there used to be. Of course, half of them go in other directions - but hopefully you'll get to benefit from some of those, too. And more nonstops are being added (the hometown airline has been announcing new European destinations recently and more are coming). So the glass is half full.
Bad news: We have far fewer nonstops to Europe than major airports in the east.
Reality: If you limit yourself to nonstops, you will be limiting your destination choices. And you'll probably be paying more, too. Lots of us have come to accept a short connection either in North America or in Europe, and it's not a terrible price to pay (the airlines charge more for nonstops - you can save money by suffering in all sorts of ways, and the airlines will offer you the chance to buy your way out of all the suffering they give you if low prices are your thing). Personally, if a nonstop doesn't work, I look for a single short connection in the western US (SFO San Francisco or YVR Vancouver are my favorites); if I can't get that, I'll take a single short connection in Europe (almost anywhere, though there are some large, chaotic airports I prefer to avoid if all things are equal). I try to maximize the long trans-atlantic flight to allow for uninterrupted sleep. The absolute worst is multiple domestic connections including east-coast ones (that makes the "long" leg too short to get useful sleep).
Try for nonstops if they work with your destinations, but don't let the airlines dictate your destinations.
From SEA, we currently have the following nonstops:
- Reykjavik (Iceland)
- Dublin
- London
- Paris
- Amsterdam
- Barcelona
- Copenhagen
- Frankfurt
- Zurich
- Rome
- Helsinki
- Istanbul
More are coming - keep an eye on Alaska Airlines.
2 - About your plans for trips of 11-12 days including travel time...I would urge you to reconsider this. A "12-day" trip to Europe gives you about 9 days of useful time on the ground. That's better than nothing, but staying longer will give you a far, far better trip if you can manage it - especially as someone new to international travel. I think the ideal length for a European trip is about 18-24 days (if you are including travel days). Coming from the west coast, I think for most people it's best to subtract 3 days in how you count full, usable days on the ground: on your arrival day, you will probably be exhausted, sleepy, disoriented, and not terribly efficient. Even on your next day - your first full day in Europe - you're going to be a little tired, jetlagged, and not operating at 100%. Your last day will be consumed with tasks required to get you on your flight home. So if your trip is N days long, you get (N-minus-3) full, usable days. Once you have a few trips under your belt, you may get more efficient (flying in business class can help too, allowing most folks to get a solid night's sleep on the way over, so you can arrive well-rested and literally hit the ground running...but not everyone can/chooses to fly that way).
In any case, I think the #1 mistake that first-timers make is to plan too aggressive of a schedule. The more I travel, the more I decide to slow-the-flock-down. It's a struggle - of course you want to see it all, do it all, you don't want to miss anything. But you gotta fight the urge to hit 18 cities in 19 days, or else your trip becomes a grinding deathmarch. Remember, every time you pick up and move from one place to another, you consume some large portion of that day in transit. More if you're flying, less if you take trains, but it all adds up.
Lots more, I'm sure the advice will roll in.
Have fun!
