Completely disagree. The airline will make a half-hearted effort to look for an I-94, but they encounter people with dual citizenship all the time. It's perfectly normal for someone to enter the US on a US passport and leave on an EU passport. That's a 5 second explanation to the ticket agent. They don't care in the slightest about legal visits to the US when you are leaving the US, only when arriving - because they have the risk of paying to deport you.

That's the result that will actually work. What's the alternative, use the expired US passport? There is zero chance the airline will let you leave the US on an expired US passport. Or any expired passport for that matter.

Use the EU passport to leave the US, which will cause no issues at all, and then deal with the situation in Spain while having all of the necessary documentation to prove US citizenship, etc.