One of those rare disagreements with Luc. Sorry, brother.
Yes. Otherwise the sugar would not dissolve completely and would be grainy. Making a syrup also allows you to add fully dissolved sugar to a cold mixture. But we're about to head South.
I believe you're wrong. Actually, undissolved sugar retards crystal formation in the same way salt on the roads prevents it. More to the point a sorbet is, in fact, a frozen syrup. This is reflected in the etymology of the word "sorbet," which comes from the latinate "sirop," which in turn comes from the Arabic "sharub." Of course, "sirop" and "sharub" both mean syrup.
Mais non! The Splenda must be completely dissolved or the sorbet will be grainy, irrespective of ice crystals. Splenda ("sucralose") won't dissolve in a cool fruit puree any better or more completely than it does in cold tea. Sucralose is sold in two different crystal forms, as three different products. One, packaged in individual packets, is made from needle shaped, easily broken crystals, and is sold for beverages. The other crystal form is packaged as a loose powder and is sold (relatively) pure for general cooking purposes, or mixed 50/50 with sucrose specially for baking. The crystals in loose packaged Splenda crystals are made from a different process than "packet" Splenda. The crystals are flat, rather than needle and less easily soluble. The beverage form dissolves more efficiently than the cooking form, which in turn, is more efficient than the 50/50. All are better in this respect than pure sugar, but none of them by much. Sorbet depends on completely dissolved sucrose (sugar) or sucralose (Splenda); and Luc's guess is DOA.
Sorbet must be made from syrup to get the right textural, weight and flavor balances.
The way to prevent ice-crystal formation in a sorbet is to stir constantly during the freezing process. That is, to use an ice cream maker. Compare this to "granita," which is made in such a way as to enhance crystal formation.
Sorbet is generally made with (wait for it) sorbet syrup (aka "thick simple syrup"), which is 2 sugar to 1 water by volume and cooked to 220F at sea level. Despite manufacturers' claim of absolute equivalency, an equivalent sucralose simple syrup is around 1-1/2 Splenda to 1 water. I didn't think it was important to go into equivalences because I thought the syrup would be used in something which was easily adjusted. But you can't taste sorbet qua sorbet until it's frozen. So, it's important to get as much of a handle on it in advance as is possible.
Because we're looking for a total dissolve in a more concentrated solution than I'd originally believed you wanted, more heat should be employed. Make by adding 1-1/2 cups Splenda to 1 cup simmering water, simmering for five minutes, then allowing the solution to cool to room temp, stirring occasionally, before using. It won't be as thick as a sugar solution, but volume for volume will be as sweet. I expect you'd end up with a better product by using the 50/50 blend and measuring and making according to normal simple syrup techniques.