Bring a large pot of lightly salted water to a boil. Cook orzo in the boiling water, stirring occasionally until cooked through but firm to the bite, about 8 minutes. Drain orzo and transfer to a large bowl.

Preheat oven to 400 degrees F (200 degrees C).

Cut a 1-inch slice from tops of orange bell peppers, discard tops, and remove seeds from peppers. Set peppers upright into a roasting pan, fitting them tightly so they stay in place.

Melt butter and olive oil over medium-low heat; cook and stir 10 cloves of garlic in the hot butter mixture until garlic is golden brown, about 8 minutes. Stir white and cremini mushrooms and balsamic vinegar into garlic. Cook and stir until mushrooms soften, about 10 minutes. Drain liquid and mix mushrooms and garlic into orzo.

Mix diced tomatoes and their juice, torn whole tomatoes, mozzarella cheese, zucchini, 1/2 cup Parmesan cheese, 8 cloves garlic, oregano, basil, red pepper flakes, salt, and black pepper into orzo mixture. Scoop into bell peppers. Pour vegetable broth into bottom of roasting pan and cover pan tightly with aluminum foil.

Bake in the preheated oven until peppers are tender and filling is cooked through, about 50 minutes. Remove foil and sprinkle each pepper with 1 tablespoon Parmesan cheese. Return to oven and bake uncovered until Parmesan cheese topping is golden and bubbly, about 15 more minutes.