The Decemberists/WMG

The Singer in The Mariner's Revenge Song Has the Wrong Man

 * The actual rake he's been hunting for years either was eaten or got away. The singer is trapped in the leviathan's stomach with an innocent man who looks vaguely (by the light in a whale's belly) like the rake; the singer simply refuses to accept the idea that he's failed in his quest.

The Rake in "The Rake's Song" is the same Rake from "The Mariner's Revenge Song"

 * Colin is specific to a fault when it comes to the chronology of both songs, I've figured out that it works very well together: at 18, the Rake seduces the three-year-old Mariner's mother, bankrupts her and gives her a fatal case of tuberculosis before fleeing. Then at 21, he marries and has four children, the last of whom, Myfanwy, is stillborn and her mother dies in childbirth. By this time he'd be about 25, assuming the pregnancies happened in quick succession, although it's possible that Charlotte and Dawn were twins. Presumably, his lack of desire to remain a father after his wife dies causes him to kill his children, and then he kidnaps Margaret. Although his children's ghosts haunt him at the end of The Hazards Of Love, it is never mentioned that he dies, so he could have gone to the sea and eventually become "the captain of [a] ship...known for wanton cruelty." Meanwhile, the Mariner is 18 when he overhears the confession in the church where he works that reveals the Rake's location to him, and he spends 20 months sailing toward him, making the Mariner 19 or 20 when they meet again, and the Rake 34 or 35—and that's where they have the duel to the death in the whale's belly after being swallowed! (copy-pasted from when I wrote it on the Just Bugs Me page)

The Mariner's Revenge Song is in the same universe as Sweeney Todd: The Demon Barber of Fleet Street.

 * It may have even been that same shipwreck that sent ol' Sweeney pitching on the waves in the first place. He was actually the lone survivor. (Dying in a whale's stomach two hours after the wreck does not count as surviving.)

The boy in Chimbley Sweep was raped and murdered by the widow.
"Upon the rooftops in dead of night/You'll hear me cry, I'll shake you from your sleep" sounds like something a ghost would say do. Being the ghost of an innocent orphaned child, he can't do much but wander and lament his fate. For bonus points, he hangs out with the cute ghost girls from Leslie Ann Levine ("On the roof above the streets/the only love I've known 's a chimney sweep").
 * Leslie Ann Levine, by the Word of God takes place in the the world of We Both Go Down Together. We might have stumbled onto something huge here.

The Rake in The Hazards of Love is Margaret's original True Love.
"My own true love went riding out" is the very first line on The Hazards of Love album, but who is singing it? It can't be William, because the singer goes on to describe Margaret and William meeting, and the singer doesn't use the first person. Margaret was in love with someone before she met William, and that man, driven mad by losing her, goes on to become the heartless Rake. Time probably flows differently in the Queen's forest, so he had plenty of time to marry and have children while Margaret was skipping about in meadows with William. When the Rake sees Margaret again, he decides to take revenge for her leaving him. When Margaret cries out "Oh, my own true love/Can you hear me love?" she's not calling to William; she's talking to the man inside the Rake she once loved.
 * I know I didn't post this, but I've developed this exact same theory. Weird. o.0 Great minds think alike, I guess.

Margaret is under a spell.
Margaret, an ordinary human girl, meets a guy who turns into a fawn, who has some kind of strange forest queen for a mother, and she hardly bats an eye. Obviously there's some magic at work here. Either the forest makes mortals go crazy, or William has (perhaps unintentionally) put a spell on her.

The Queen, not Annan Water, drowns Margaret and William.
William promised himself to the river in an attempt to save his own true love. The Annan was so impressed by William's bravery and willingness to sacrifice himself that it let him go free. The Queen, on the other hand, decided to punish William and Margaret, and drowned them herself. The Queen of the forest could easily have saved her son if she wanted to, and she could just as easily have made the river swallow him up.

Many of the band's songs are connected, not just from the concept albums.
It's been confirmed that Leslie Ann Levine's mother is the girl from We Both Go Down Together, and we've possibly hit a connection with The Chimbley Sweep as well ("On the roof above the streets/The only love I've known is a Chimbley Sweep"). Is it possible there is an overarching canon to some of the songs?
 * The last line of "Sons and Daughters" is Here all the bombs fade away. On the same album, we have a song called "After the Bombs."
 * "Sons and Daughters" actually fades into "After the Bombs," which makes this even stranger. Also, when you have "The Crane Wife" playing on repeat, the last line of the album is "Until it all starts over again." At which point, it all does start over again.
 * I can't imagine that that was their intention, as After the Bombs was an iTunes only bonus track. There were in fact five different versions of the album, each with a different final track, and one with none at all.
 * Now that I think of it, we have "When The War Came" as well...

Both Margret and William are Forest Spirits
William is a seed trapped in an area of the forest that has bad soil ("Trapped in a cradle of clay"), and the Queen of the forest dug him up and gave him a dual Faun/Human form. Margret is in fact the daughter of the river spirit (Annan Water) whom fell in love with William, against the wills of both parents. Making this less of a fantastic pantomime than a straight-up Romeo and Juliet story with fantastic elements.

The Hazards of Love isn't about the story, but rather the telling of the story
If you take a step back and consider it from a Meta point of view, as a kind of play or musical, some elements start to make more sense. The entire story is very Shakespearean, and if you consider the characters from the point of view as players on a stage, some of the odd bits start making sense: the reason all the characters are human, for example, is because it's impossible to have singing and dancing trees and water.

The Legionnaire is rescued at the end of The Legionnaire's Lament
The final lyrics are "I'll Be Back Again", so one assumes that he's going back home.

'Collin Meloy is a psychopath.
This isn't so much of a theory than a literal interpretation of the lyrics.
 * There is fanfic. And it's awesome.

The Crane Wife is a Chinese woman married to a Japanese businessman
And the songs referring to the fable are just an allegory/allusion to the original story when the husband recalls the story after an aggravating, drawn-out divorce.

This mentioned combination most common in the types of inter-racial marriages involving Japanese citizens. Often there's the element of economic inequality involved.

The bride on the cover of the album is wearing Manchu-style clothing whereas the groom is wearing a business suit, which, applying the recent history between the two countries, is apt. The crane referred to is most likely a red-crowned crane, which migrates between the two countries. Oh, and her wedding dress is white with a red splotch where the arrow struck her.

The whale from The Decemberist's "The Mariner's Revenge Song" is the ghost of the protagonist's mother
The protagonist mentions hearing her after her death; the whale appears after the protagonist hears her and gives her son a chance at taking revenge into his own hands.

The narrator of The Decembrist's "The Mariner's Revenge Song" is on the Pequod and chasing Moby Dick.
The ship does go down. Along the way, Queequeg nearly dies and orders a coffin to be made by the ship's carpenter, which he then decorates with carvings of his own design. After the ship breaks up, the narrator finds it floating and uses it like a canoe. (Oh, and Ahab harpoons himself to the whale.)

The narrator of "We Both Go Down Together" by the Decemberists is the 'lad of eighteen' from "The Mariner's Revenge Song"
The narrator is callous, handsome, and deluded. He would rather believe that he's in a tragic love affair with a lower class girl than admit that he's a crazy rapist. Now the girl's pregnant, and he's going to push her off a cliff and scarper. He'll do it romantically, of course. (The baby is Leslie Anne Levine, but that's another story.)

The Rake actually killed Margaret before William/his dead children got to him.
Now, I'm only going by the album version here, as I've never seen it. But in the last two songs, we only hear Margaret's voice either alongside William's, or as a wail in the background. You'd think that, if she were still alive, she would have something to say during "The Drowned," at least. There are other lines, like "painting rings around your eyes" that make it seem like he's watching her while she's lying still. Too still.

So here's my WMG - William gives up and doesn't try to escape the fortress and the water not just because he's got a code of honor that won't let him renege on the promise that he made in "Annan Water," but also because his beloved and their gestating child are both dead. He's made an enemy of his own mother, and his beloved Margaret is dead, taking with her their hope for a family. He's lost the will to fight anymore. "The Drowned" is William trying to get her to wake up by calling her name, trying to get her to respond. He realizes that she's not going to wake up and he's going to die sometime around when he suggests that they declare themselves married there. The times when we hear Margaret's voice alongside William's is easily explained as her spirit still lingering because he's going to die soon, and him slowly drowning and losing his grip on the mortal world (and sanity).

The crime in "The Perfect Crime" was an elaborate bank heist.
It was a massive effort to get the contents of a safe. But they disguised it by kidnapping a businessman's daughter and threatening to assassinate the businessman. While law enforcement was busy dealing with the hostage situation, they robbed the bank. They still got caught, though, if the lyrics are anything to go by.

"Here I Dreamt I Was An Architect" is about a couple who are repeatedly reincarnated.
Every time they're reborn, they lose their relationship for some reason and keep coming back - "and try one, and try two/ I guess it all comes down to / Alright, okay".

The Architect and the Engine Driver are the same people.
Both songs describe figures that can't figure their lives out, or have a longing desire for something more. Seems similar, but not connected, until you learn the chords - "Here I Dreamt I was an Architect" has two distinct chords that go with the riff, and "The Engine Driver" starts with the exact same two chords.