Put a splash of the Lime-Mint Simple Syrup in one saucer and the crushed graham crackers in a second saucer. Dip each highball glass first into the syrup and then into the graham crackers. Add a scoop of the lime sorbet to each glass.
Add the remaining Lime-Mint Simple Syrup to a pitcher with the seltzer. Slowly fill each glass with the seltzer mixture to within 1 inch from the top. Top each with 1/4 cup of the whipped cream and some lime zest. Serve with a straw.
Combine the sugar and 1 cup water in a small pot and cook over medium heat until the sugar is dissolved, a few minutes. Stir in the lime zest and juice and remove from the heat. Add the mint leaves and let steep for 10 minutes.
Strain to remove the mint leaves and refrigerate until ready to use. Keep any remaining syrup covered in the refrigerator for up to 2 weeks.