It was very difficult to find a flight from Lisbon to Athens (see SWISS Business: Why Am I Leaving Lisbon for Geneva?). It was even more difficult to find a flight back. To make it work, I had to book a business-class flight on Iberia, then a coach flight on TAP. I departed at 3:50 PM and arrived in Madrid at 6:45 PM. From there, I was stuck in the airport until 11 PM.
Residency Lesson: On residency, only direct flights are allowed. The point is to get in, get out, and do as little as possible to disrupt my schedule.