This is a great website! Thank you all for your thoughtful replies. After investigating it looks like Seattle to Rome through Heathrow is the best option. The layovers are only about 1.5 - 2 hours long. There are 6 flights from Heathrow to Rome, so if she missed the connection, she has more options to get to Rome. Returning, there are two flights from Heathrow to Seattle. I checked the British Airways website and both the inbound from Seattle and outbound to Rome arrive in Terminal 5.
I've searched all your recommendations but could find no other airline that met all the criteria.
The Air Transat nonstop Vancouver to Rome looked like a great deal and perfect - until I read the deplorable reviews. I cannot risk an airline with such undependable service.
The Italian consulate said that since she is 16 she would only need her passport.
Any other comments would be appreciated. Using websites and finding flights seems easy - it's the things they don't tell you that I worry about!