Whisk together mustards, vinegar, mustard seeds, and shallots. Add salt, to taste.
Grind the rice in a blender until very fine. In a bowl, stir together the ground rice, semolina, flour, salt, and pepper.
Heat several inches of oil to 375 degrees F in a deep fryer or a wide, deep, heavy pot. Preheat the oven to 200 degrees F.
Slit the calamari bodies open so they lie flat. Place calamari (bodies and tentacles), mushrooms, fennel, and lemon in separate bowls. Toss each with enough buttermilk to moisten them. Dip the calamari generously in the rice batter, using enough to fully coat.
Working in batches (it's important not to overcrowd the pot), fry until golden brown and crisp, and then transfer to a tray lined with paper towels. Season immediately with salt. Transfer to a low oven to keep warm. Repeat with remaining calamari.
Coat and fry the mushrooms, fennel, and lemon slices in the same way, working in batches and transferring them to the low oven as they are done.
Put a generous pool of mustard sauce on each plate, then top with an assortment of fried squid, mushrooms, fennel, and lemon.