Elektrotek - I don't really have a whole lot to add to what kamikaze and gweilo already said, except that lodging somewhere within walking distance is probably about a hundred times more preferable to staying at Echigo Yuzawa and commuting by shuttle bus everyday, which is a colossal pain in the ass by all accounts. Getting assistance from someone who speaks Japanese will probably make things a lot easier for you - most of these inns are small mom-and-pop type of operations, and I don't imagine they have many staff members who are fluent in English.
I definitely wouldn't be surprised to see Lily Allen added to Fuji Rock, but a headlining slot would be a pretty big stretch IMO. A subheadliner? Maybe. As for Interpol, they've played Summer Sonic before so why not?