For the risotto: Heat a 3 1/2-quart Dutch oven over medium-high heat. Add the oil and butter. When the butter is melted, add the shallots and garlic; cook, stirring often with a wooden spoon, until the vegetables are soft and fragrant, about 2 minutes. Add the salt and the rice; stir to coat with the flavors of the pan. Cook for 1 minute, then deglaze with the white wine. Reduce the heat to medium and simmer, stirring often, until the wine is almost fully absorbed. Add the broth and stir to combine; cook, stirring often, until the rice is tender but not mushy, 15 to 20 minutes. Remove from the heat and stir in the Parmesan, mascarpone, lemon zest, crab and chives. Spread the risotto out on a parchment-lined baking sheet and let cool to room temperature, then cover with plastic wrap and refrigerate until completely cold.
For the arancini: Prepare a breading station using three shallow bowls: Place the flour and salt in one bowl, the eggs in another, and the panko in yet another. Using a 1-tablespoon ice cream scoop or spoon, scoop a round of risotto into the palm of your hand and form it into a ball. Roll the ball in the flour, then the egg, and then the panko; make sure the entire ball is well coated. Place on a rimmed baking sheet and repeat with the remaining risotto.
Add enough oil to a medium saucepan to come 2 inches up the side of the pan. Heat over medium-high until the oil reaches 350 degrees F on a deep-fry thermometer. Using a spider or slotted spoon, lower 5 or 6 rice balls into the oil, or as many as the pan will allow without crowding. Fry until deep golden brown and heated through, 3 to 4 minutes. Remove to a paper-towel-lined baking sheet to drain. Continue with the rest of the rice balls. Serve warm, with marinara alongside for dipping, if desired.