Rub the corn with 2 teaspoons olive oil. Grill, turning occasionally, until lightly charred, about 5-7 minutes. Let the corn cool slightly before slicing the kernels off the cobs.
Grill the lemon halves for 1-2 minutes, cut side down, until they are grill marked.
For the couscous, heat 1 tablespoon olive oil in a large saucepan over medium heat. Toast the couscous stirring until it golden, about 5 minutes. Add the water and bring to a boil. Reduce heat to low, cover and simmer, stirring periodically, until the couscous is tender and most of the water has been absorbed, about 10 minutes. Drain any remaining water.
In a large bowl, combine the couscous and corn. Squeeze in the grilled lemon juice and fold in the scallions, walnuts, feta, parsley, salt and pepper.
Season to taste with flaky sea salt and additional black pepper.