Combine shallot, vinegar, and lemon juice in a small bowl. Season with salt and let sit 20 minutes to lightly pickle. Using a spoon, scoop avocado flesh into a food processor; discard skins. Puree until smooth. Add preserved lemon, yogurt, oil, and shallot along with pickling liquid; season with salt and pulse to combine. Transfer avocado mixture to a medium bowl and stir in herbs. Taste and adjust seasoning with salt and/or more lemon juice, if needed. Herbed avocado can be made 1 day ahead. Cover, pressing plastic wrap directly onto surface, and chill. Whisk lemon juice and agave in a medium bowl; season with salt. Whisk in grapeseed oil, then olive oil. Taste vinaigrette and adjust seasoning with more salt if needed. Vinaigrette can be made 5 days ahead. Cover and chill. Toss lettuces, spicy greens, herbs, and edible flowers in a large bowl to combine. Drizzle about half of lemon vinaigrette over and toss to coat. Season salad with salt. Spoon 2 dollops of herbed avocado onto each plate and loosely mound salad over top. Scatter pistachios over and around salad and drizzle with remaining lemon vinaigrette.