I'm very sorry to have been confusing! To me, the flight into Seattle was inbound and the flight out of Seattle was outbound. Especially since they were not part of a round trip flight. I did not even realize there were specialized meanings, thank you for the explanation.
You are correct, the two legs have different 3 digits (006 for Delta, 016 for United). That's helpful to know.
My information comes from (1) how generally unhelpful EZ Air was when I inquired about changing flights - hypothetically - and (2) online chat with Princess today - you're right, I should take that with a grain of salt. They told me to call EZ Air tomorrow but also said it was a round trip and canceling one would cancel the other flight. Very discouraging.
I can see where United might have no idea that I've missed the Delta flight. But EZ Air would know - and I guess they could cancel the United leg?