Make the apples: Toss 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 dissolves 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. Repeat ice cream, apple, and crumble layers. Top with whipped cream.
The three components -- ice cream, caramelized apples, and crumble -- can be made ahead for entertaining. Simply assemble just before serving.