Have you thought of using BA? Their should be a flight that works for the outbound that doesn't require a change of airport to FCO. The ability to do a stopover has been our big reason to use BA as our award airline for Rome.
Can also train it to Venice from Termini or v.v as Missy stated.
AA also has decent service (seasonal) from ORD and JFK to Rome. Would highly suggest ORD routing for Seattle routings.
lala