Make the Apples: Toss the apples with lemon juice in a large bowl, and let stand. Melt butter in a large skillet over medium heat. Add sugar, and stir until sugar dissolved and mixture is dark amber, about 15 minutes.
Stir in apples, their juices, and the salt. Reduce heat to medium-low, and cook, stirring occasionally, until apples are softened, about 10 minutes. Stir in Calvados and cream. Transfer apple mixture to a bowl.
Prepare an ice-water bath. Set the bowl in the bath and let apple mixture cool, stirring occasionally, until mixture is cold and sauce has thickened, about 20 minutes. Use immediately or refrigerate overnight.
To Assemble: Whisk cream to medium peaks. For each serving place 2 caramelized apple wedges in a sundae glass. Top with a scoop of cinnamon ice cream, 2 caramelized apple wedges, and some pecan-oat crumble layers. Top with whipped cream.
Cinnamon Ice Cream: Heat milk, cream, and vanilla seeds and pod in a medium saucepan over medium heat until bubbles begin to form around edges. Remove from heat, cover and let cream mixture steep at room temperature for 1 hour.
Prepare an ice-water bath. Uncover cream mixture, and reheat until hot but not boiling. Whisk egg yolks, sugar, cinnamon and a pinch of salt in a large bowl. Add hot cream mixture in a slow steady stream, whisking constantly, until well combined. Return mixture to saucepan, and stir constantly over medium heat until thick enough to coat the back of a spoon, about 8 minutes. Do not let the mixture come to a simmer.
Strain mixture through a fine sieve into a bowl. Discard vanilla pod. Set the bowl in the ice-water bath. Let cool, stirring occasionally, until cold, about 20 minutes.
Freeze mixture in an ice cream maker according to manufacturers directions. Transfer to a large container, and freeze until firm, about 1 hour. Ice cream can be frozen for up to 1 week.
Pecan-oat Crumble: Preheat oven to 325°F Cream butter and sugars until smooth. Add pecans, oats, flour, baking powder, cinnamon and a pinch of salt. Stir until mixture begins to form large clumps. Transfer mixture to a rimmed baking sheet, and spread in a single layer. Clumps should be close together.
Bake until golden brown, about 25 minutes. Let cool completely, on sheet on a wire rack. Using your hands, break into small pieces. Crumble can be stored for up to 1 week.