Toss the crab, 2 tablespoons herbs, half each of the lemon zest and juice, and salt and pepper to taste in a bowl; cover and refrigerate. Whisk the remaining lemon zest and juice with the olive oil in a separate bowl. Toss 1 tablespoon of the lemon oil with the bread cubes; season with salt and pepper. Heat 2 tablespoons butter in a large skillet over medium-high heat. Cook the bread cubes until golden, about 7 minutes. Drain on paper towels.
Heat the remaining 2 tablespoons butter in a pot over medium heat. Add the leeks and cook until wilted, about 5 minutes. Add the potatoes and fennel and cook, about 7 minutes. Stir in the wine, 2 teaspoons salt and 1/4 teaspoon pepper; cook 2 more minutes. Add 5 cups water and 1/2 cup herbs. Bring to a boil over medium-high heat, then reduce the heat to medium low and simmer until the vegetables are tender, about 20 minutes.
Puree the soup in a blender in batches. Return to the pot, add the cream and simmer until thick, 5 to 10 minutes. Add the remaining 6 tablespoons herbs; season with salt and pepper. Ladle the soup into bowls and top with the crab, croutons and the remaining lemon oil.
When blending hot liquids, first let cool for 5 minutes or so, then transfer to a blender, filling only halfway. Put the lid on, leaving one corner open; Cover the lid with a kitchen towel to catch splatters and pulse until smooth.