Add sweet potatoes and water to a 6-qt Instant Pot®. Select manual setting; adjust pressure to high, and set time for 8 minutes. When finished cooking, quick-release pressure according to manufacturer’s directions.
In a medium bowl, combine pecans, 5 tablespoons butter, 1/4 cup brown sugar, flour and 1/2 teaspoon salt; place in the refrigerator.
Remove potatoes from the Instant Pot® and drain excess liquid before returning the potatoes to the pot. Using a potato masher, mash potatoes until smooth.
Stir in remaining 5 tablespoons butter, remaining 1/2 cup brown sugar, eggs, milk, orange zest, vanilla, cinnamon and 3/4 teaspoon salt.
Preheat oven to 350 degrees F. Lightly oil a 9 x 13 baking dish or coat with nonstick spray.
Spread potato mixture into the prepared baking dish; sprinkle with pecan mixture.
Place into oven and bake until almost set, about 40 minutes. Sprinkle with marshmallows; continue to bake for an additional 10-15 minutes, or until marshmallows are golden brown.