For the pie: Combine apples, sugar and salt in colander set over large bowl. Let sit, tossing occasionally, until apples, release their juices, about 30 minutes. Press gently on apples to extract liquid and reserve ¾ cup juice.
Adjust oven rack to lower middle position and heat oven to 350 degrees.
Pulse crackers and remaining sugar in food processor until finely ground. Dust work surface with cracker mixture, brush half of one pie round with water, overlap with second pie round, and dust top with cracker mixture. Roll out dough to 19 by 14 inches and transfer to rimmed baking sheet. Brush dough with butter and refrigerate; roll out top crust in the same way.
Toss drained apples with tapioca, cinnamon, nutmeg and lemon juice and arrange evenly over bottom crust, pressing lightly to flatten. Brush edges of bottom crust with water, and arrange top crust on pie. Press crusts together and use a paring knife to trim any excess dough. Use fork to crimp and seal outside adge of pie, then to pierce top of pie at 2-inch intervals. Bake until pie is golden brown and juices are bubbling, about 1 hour. Transfer to wire rack and let cool.
For the glaze: While pie is cooling, simmer reserved apple juice in saucepan over medium heat until syrupy and reduced to ¼ cup, about 6 minutes. Stir in lemon juice and butter and let cool to room temperature. Whisk in confectioners’ sugar and brush glaze evenly over warm pie. Let pie cool completely, at least 1 hour longer.