Phew, we tested negative, but what a stressful way to end a month in Italy.
Once we learned we couldn't test in Spello, we looked at FCO testing and our prebooked train and realized we'd only have about two hours to test before they shut down for the day at 1600 the day before our flight. So, rather than the noonish train, we bagged our tickets and bought new tickets for 0630 instead to make sure we had time for however the testing took.
Testing in terminal 3 is easy to find, but plenty of what-the-heck-do-we-do-now. No info, fill out a tablet screen, get in line, grab one, two, three, four forms each. Find test a reservation number on our cell phones, figure what test (antigen or pcr), sign forms, agree to be lodged at 98euros a night if positive, get in line again, pay and wait, get tested and wait, and if lucky, negative.
About an hour all told. But, jeez, stress.