You are fine on the outbound - but the airline will check your passport on check-in on the return, and may not let you fly on an expired passport (they are liable for ensuring that all pax meet entry requirements, and may be fined if they try and bring in someone who doesn't). Now you could get around this by showing the UK passport - but if they know you are a US citizen, then they know you have to enter the US on that passport.
Going for the renewal in Spain seems to be the best way forward.
I hope your dad is ok!