We recently went on a RS tour of Sicily. When I searched for flights from Sea-Tac to Sicily there did not seem to be any reasonable options as there were extremely long layovers between connections. I checked out flights from the airport in Vancouver BC and found a much more reasonable connection on United/Lufthansa - direct, non-stop to Munich, about a 2 hour layover in Munich and then a direct, non-stop to Palermo. We rented a car and drove to Vancouver and this worked well for us .We live north of Seattle and a drive to Vancouver would allow us to avoid navigating Seattle traffic. It helped that we had a Nexus pass and got through the border without any delay. The rental car return is less than a minute walk to the international departure terminal. Security was a reasonable process. We were quite happy using this alternative to Sea-Tac. A one-way international car rental is not cheap but for us it seemed a reasonable option.
Happy travels!