First, to the OP: Is this a recipe you’ve made successfully before? If not, here’s a standard (non chocolate) chess pie recipe for comparison: http://find.myrecipes.com/recipes/recipefinder.dyn?action=displayRecipe&recipe_id=257879. This recipe IS for a 9-inch (deep dish) pie whereas yours looks to be proportioned for an 8-inch (standard) pie, so the ratios are more important than the actual ingredient amounts.
Second, “Pet milk” is NOT the same as sweetened condensed milk. What is sometimes called Pet milk is evaporated milk – not sweetened and not nearly so concentrated as sweetened condensed. I don’t know which your recipe is really calling for, but with 1 1/2 cups sugar I’m guessing it’s the evaporated milk you should use.
Third, there are two versions of Splenda for use in baking: One is 100% Splenda and can be subbed measure-for-measure, one-for-one with sugar. So, if a recipe calls for 1 cup sugar, you can sub 1 cup of Granulated Splenda. There is also a version called for baking called Splenda Blend. It consists of 1/2 Granulated Splenda and 1/2 sugar. It is NOT a 1-for-1 sub with sugar, so read the measuring guidelines. (It’s also more expensive than just buying Granulated and mixing with sugar.) Note that if you are making something where sugar is there for structure as well as for sweetness (any type of cake/cookie/bread/etc. that will “rise”), you CANNOT use the Granulated Splenda alone, you should sub no more than half the sugar with the Splenda. There’s lots of good info on the Splenda web site about how to bake with it, but it shouldn’t be a factor in this pie.
As for making this recipe lighter … there are a few things you can do, but you may have to experiment to get a level of compromise that satisfies you. You can get low-fat or skim evaporated milk (or sweetened condensed, if that’s what you’re going to use). I would try looking specifically for whatever “healthy margarine” you want to use that’s made especially for baking. One of the problems with light margarines is that they cut calories by adding water to replace fat. You might be better off using real butter but less of it. You only have 2 eggs, you could try 1 whole egg plus 2 egg whites to cut out one yolk. Or, you could try an egg substitute. Remember, too that your pie crust contributes a significant amount of fat and calories to this dessert. “Light” pie crusts usually don’t work, though, because they are so dependent on the high fat content for texture.
Here’s a Cooking Light recipe for a lemon chess pie that might give you some ideas: http://find.myrecipes.com/recipes/recipefinder.dyn?action=displayRecipe&recipe_id=223007. CL does not use sugar subs, so you could still use Splenda if desired to reduce a little more.
One of the things I’ve found with lightening desserts is that if you make too many changes, the end result just isn’t something you want to eat, even if you are used to eating “light” food. I would decide what you really want to lighten – if it’s just calories, I’d cut the sugar and maybe use the low-fat milk but leave the rest. If fat’s what you really want cut, leave the sugar alone. Also, remember that portion size can be the most important factor in cutting down.