For the filling: Preheat the oven to 400 degrees F.
Bring the sugars, cream of tartar and 2 cups water to a boil in a saucepan over medium-high heat, whisking thoroughly as the sugar dissolves. Add the butter, cinnamon, nutmeg, lemon zest and lemon juice and whisk until the butter is melted. Boil for 5 minutes. Remove from the heat and stir in the vanilla.
Put the crackers in a heatproof bowl and pour the syrup over the top. Let mixture sit for 5 minutes, stirring occasionally. Pour into the pie shell.
For the topping: Mix the crushed crackers, brown sugar, melted butter and cinnamon in a small bowl. Sprinkle over the top of the pie. Set the pie on a rimmed baking sheet and bake for 10 minutes, then lower the temperature to 350 degrees F and continue to bake until the top is golden brown and the filling is set, 18 to 20 minutes. Allow the pie to cool slightly.
For the faux whipped cream: Whip the chickpea liquid with an electric mixer until bubbles begin to form. Add the cream of tartar and continue to whip on high speed until peaks form, 2 to 3 minutes. Pour in the vanilla and continue whipping for another 1 to 2 minutes. Slowly add the confectioners' sugar while whipping an additional 3 to 5 minutes.
Dollop slices of the pie with the faux whipped cream and serve immediately (see Cook's Note).
Faux whipped cream must be served immediately or it will fall and separate. If this happens, simply whip it again for a few minutes to bring it back together.