Step 1Quarter the strawberries, spread them in a single layer on a baking sheet and freeze them until they are almost solid, 20 to 30 minutes.
Step 2Place the berries in a food processor with the sugar and orange zest. Pulse to chop the fruit, then process continuously until the berries are pureed. Add the rum and preserves. Process briefly to mix. With the processor running, add the cream through the feed tube in a steady stream. Process until just mixed. Transfer the mixture to a plastic container and freeze until it's firm enough to scoop, about 2 hours.
Step 1Whip the cream and sugar until thick. Transfer it to a bowl. Whip the ice cream until it's thick and pourable. Fold it into the whipped cream along with the rum and Cointreau.
Step 2Use a 1/4-cup scoop for sherbet, putting 1 scoop in each of 6 stemmed glasses. Top each with 1/4 cup of sauce. Add another scoop of sherbet and then more sauce to cover. Place the sundaes in the freezer several hours or overnight, covered airtight.
Step 3To serve, let the sundaes rest at room temperature to soften slightly, 30 to 40 minutes. Top them with sliced strawberries, if using. Serve immediately.