Pulse graham crackers in a food processor until finely ground, 10 to 15 times. Add melted butter and 1/2 teaspoon salt; pulse until crumbs are evenly moistened, about 5 times. Transfer crumb mixture to a 9-inch springform pan, and press onto bottom and about 1 1/4 to 1 1/2 inches up sides, tapering top edge as you press up sides. Freeze 30 minutes. In a clean food processor, combine sliced peeled peaches, 6 tablespoons agave, and 1/2 teaspoon salt; process until smooth, about 2 minutes. Transfer to a large bowl, and refrigerate until chilled, about 30 minutes. In a clean food processor, combine cream cheese, ricotta, lemon juice, sugar, remaining 6 tablespoons agave, and remaining 1 teaspoon salt. Process until very smooth, about 2 minutes, stopping to scrape down sides of bowl as needed. Transfer to a second large bowl. Spread half of cream cheese filling in prepared crust in an even layer; freeze 45 minutes. Chill remaining cream cheese filling (do not let it become too firm). Carefully spread half of chilled peach puree in an even layer on top of cream cheese filling in pan; freeze 45 minutes. Carefully spread remaining cream cheese filling in an even layer on top of peach layer; freeze 45 minutes. Spread remaining peach puree on top. Cover pan with plastic wrap; freeze until firm but not completely frozen, 3 to 4 hours. Remove sides of pan; transfer cheesecake to a serving plate. Top with unpeeled peach slices.