Originally Posted by
frankvb
but you can select premium seats without any additional fee during the booking process
Interesting. I'd only come across results that required payment for Premium seats in this scenario (Q and O class). Playing around with different dates & times, I finally found one with that did offer the Premium Upgrade without extra payment (S class).
But the other thing is that MKG/SEA searches present me with mostly crappy results, including lots of "+1 day" options. It looks like I'll come out ahead overall just booking MKG/ORD and ORD/SEA on separate itineraries.
I do still wonder if the types of search results I encountered are by design, though.