You can also travel by train to Naples. From Milano Centrale it’s about 4.5 hours. See what your options are. From Milan Malpensa MXP to NAP the only non stop options are with EasyJet or Ryanair. Both have one flight super early in the morning, and one flight in the afternoon at around 4pm. Check prices, times, and see what works best for you.
To fly home you might want to consider taking the train to Rome the day before your flight to the US. Only ITA airways flies to Rome from Naples, and only with 3 flights a day, whereas the trains are plentiful. Another option, instead of returning from Rome, is to fly directly from Naples NAP to Portland PDX via some other European or US gateway. For example you could fly Naples NAP>London LHR>Portland PDX with British Airways, or also Naples NAP>Newark EWR>Portland PDX with United Airlines or Naples NAP > New York JFK > Portland PDX with Delta Airlines. Some of the aforementioned flights are seasonal, so they may or may not be available for you depending on when you travel. Lufthansa also flies to Naples from Frankfurt and I think Condor has a seasonal flight from Frankfurt to Portland PDX. Naples is not a minor airport, so there are many options you can explore without having to go to Rome. You just have to do some research on Kayak or Google Flights.
