Preheat oven to 400°. Spray 2 baking sheets with canola oil spray and arrange 4 tortillas on each. Spray tortillas with more oil, then bake until golden brown and crisp, about 8 minutes. In a medium bowl, mash together avocados, shallot, 3 tablespoons lime juice, and cilantro, and season with salt. In a large bowl, whisk together cumin, cayenne, 2 tablespoons lime juice, and 2 tablespoons olive oil. Add shrimp and toss lightly to coat, then season generously with salt and pepper. Grill shrimp over medium-high heat for 3 minutes per side, or until bright pink and cooked through. In a small bowl, toss cabbage and carrot with remaining 2 tablespoons lime juice and 2 tablespoons olive oil. Season with salt and pepper. Arrange tostadas on a flat work surface and spread with guacamole. Top with shrimp, slaw, and cilantro leaves and serve with lime.