1a. Use your EU passport at the ticket counter in the US.
1b. Then, use your EU passport at wherever you land in Europe
2a1. Use your expired US passport at the ticket counter in Spain.
2a2. If you catch guff from the ticket counter in Spain about your expired US passport, then use your UK passport.
2b. Use your expired US passport at Immigration in the US.
Now, when are you leaving, and where do you live? You can get a passport in a few hours if you can get to a US Passport Office. You can also get a renewal very fast at a US embassy/consulate, so it's not like you have to take a day to get it done once in Europe.