Any idea on the difficulty of this, beyond the stress from their hatred for crates?  We are traveling from the mid-west of USA, with two cats.

Hello

I brought my cat from France by plane, he took pill and everything went well, after I make the european vaccination before departure and it must have a European health card, it traveled with me on the plane at my feet without any problems he slept, customs and everything looked no problem, you should talk to your veterinarian who knows all the legal paper, vaccinations, travelling methods…

Good trip

Yes I just brought my two cat over from the US East coast in May. It was a pain, but not impossible. How well do you speak Spanish?
There is a Madrid Vet that is originality from Nebraska that is supposed to be able to help with the Spanish Customs. She is a little pricey and we opted not to use her.
Fell free to PM me with questions.