For the calamari: Roughly chop 3 of the calamari tubes and add to a food processor with the shrimp and garlic. Pulse into a coarse paste. Remove to a large bowl and add the spinach, eggs, breadcrumbs, cheese, herbs, and crushed red pepper. Season with salt and stir to combine. The mixture should be a little chunky but moist and holding together. Test the seasoning of the mix by making a mini patty, cooking it in a small pan, and eating it. It should taste really good! If it doesn't, it is probably missing salt. Add more salt, as needed. Divide the stuffing among the remaining 8 calamari bodies and secure closed with toothpicks.
For the sauce: Coat a large, wide skillet or saucepan with olive oil and add the onions and crushed red pepper. Season with salt and put the skillet over medium heat. Saute the onions until they are soft and aromatic, 7 to 8 minutes. Add the garlic and cook until fragrant, another 2 minutes. Stir in the wine and cook until the wine has reduced by half. Pass the tomatoes through a food mill or puree them in a food processor and add them with 1 cup of water to the skillet. Season with salt, to taste, and simmer the sauce for 15 minutes.
Add the calamari to the skillet and cook for 20 to 25 minutes, turning them over halfway through the cooking time. Remove the calamari from the skillet and remove the toothpicks. Reduce the sauce to thicken, if needed. Arrange the calamari on a serving platter and cover with the sauce before serving.