Looking at actual itineraries, I see that the flights from Seville to Seattle with only one connection take 25-30 hours because they require an overnight somewhere. For the One World group (British Airways, American, Iberia, etc)., that overnight appears to be in London. For the other airlines (Lufthansa, Air France, Delta, etc.,.) the overnight could be in Frankfurt, Paris, or possibly elsewhere.
The other options are all flights with two connections, and given how chaotic and uncertain air travel has become this summer, I would strongly advise against such an itinerary. Too many ways that could go wrong.
So it looks like you will have an overnight somewhere, no matter what. You may as well take control and decide where you want that to be.
I was unable to find any direct flights from Madrid to Seattle, but perhaps someone else has better ways to search than Google flights. If not, the route by train to Madrid and flight home from there is not a bad choice even though you have to make a connection somewhere. You could depart Seville the morning the tour ends, and have the afternoon and evening to spend in Madrid, overnighting there (we stayed in a nice hotel within walking distance of the train station and the airport bus). You’ll get a good night’s sleep and have a more enjoyable, all-daytime journey home to Seattle the next day.