I think that Black Sandy Beaches is an unknown narrator who found their letters in a bottle somehow and just speculates about it. It's an interesting perspective. Dear Ms. Leading is the last one where he actually talks about her. It's the actual letter. At least that's how I always interpreted it. VVV is definitely about him going to the war, though. He gets on the boat to leave in that song, I'm pretty sure.
Yup. Correct on all counts. (Although I think there are multiple letters, indicated by lines like "In response to your response" in Dear Ms. Leading and "further steps reveal another broken bottle" in Black Sandy Beaches).
I consider VVV epic because not only the song a perfect wrap-up of this chapter to the story, but the title itself is such a fitting description of the ending. I.e., the only way he can feel freedom is by starting a new life (hence the vessel vindicating - although, I guess the 'vindication' may also be the fact that now Ms. Leading is the one who is heartbroken, as she watches the ship depart with the only man she's ever loved), and the only way he can start a new life is by hopping on the ship (hence the vessel being vital). The Boy has had a horrible time and has had his heart shattered and, despite the warnings of the Oracles ("If you flee from grace your souls cannot be saved - big steam ships, exits illustrate the flaw - don't be ashamed of your amore faux pas and when the bombs go off you'll know right where you are"), he feels the only way he can get away from it all is by running away ("but the chances of escaping my heart are inadequate"), and that the Oracles were wrong ("the exit's unflawed - the boys on the train - the almighty tongue with prose spilled in vain").
And I love the interplay between the lyrics of Oracles and of VVV. In Oracles he's told "You've been stuck in the middle of patience and animosity", but by the time he finds freedom on the vessel, he's come to the conclusion that he's finally passed that when he says "Goodbye, my eyes shed heavy tears - one for every soul still sitting on the fence between pain and arrogance".