The flights both to and from SEA look fine .. over 50 empty seats. BA560 on the 14th is tight with about 10 empty but the later 556 has over 50 empty. The VCE-LGW flight is OK with over 30 empty seats.
As mentioned driving in Italy is categorized as a dangerous sport so I'd suggest the train or fly internally ( as the train is about 5 hours and the flight 1 hour ) depending upon how much time you have.
You can check out the Italian train times and prices at -
<a href="http://www.trenitalia.it/en/index.html" target="_blank">http://www.trenitalia.it/en/index.html</A>.
( check out the EUR 29 specials on the Eurostar trains )