As a general note - if you're not an experienced baker, I would *strongly* recommend either finding an already healthified recipe, or cooking the recipe as listed first, before making substitutions. Baking is much more sensitive to substitutions than regular cooking and it's very easy to mess up the chemistry. With experience you learn what substitutions work, and what don't.
For example - I know that if I substitute brown sugar for white in a chocolate chip cookie recipe, I'll get really chewy cookies, which I like. If I do the same with a shortbread recipe, I won't get shortbread.
1 cup all-purpose flour
1/3 cup granulated sugar
1/4 cup cold margarine or 1/4 cup butter
3 tablespoons low-fat yogurt
1 -2 tablespoon cold water (15 or 25 mL)
This is very different than pie crust recipes I'm used to - what I'm used to would have about 1/2 cup fat to 1 cup flour, but no sugar or yoghurt. I suspect this is a low fat recipe, hence the yoghurt and sugar additions. It won't be as flaky as a standard pie crust, because the fat is what makes it flaky.
Brown sugar isn't any more low calorie than white, but it does change the texture of baked goods considerably as it has more moisture than white, so it generally making things moister and more chewy. This can be great in chocolate chip cookies, less so in pie crusts.
1/4 cup freshly squeezed lime juice
0.25 (50 ml) fresh lemon juice
1 1/2 teaspoons grated lime zest (about 2 limes)
1 1/2 teaspoons grated lemon zest (about 1 lemon)
1 egg white
1 1/3 cups granulated sugar
1 1/4 cups water
1/3 cup cornstarch
2 teaspoons margarine or 2 teaspoons butter
This looks like a pretty standard lemon pie filling, and I don't think you can fiddle with this much. You need the sugar to offset the sourness of the lemon juice (brown will significantly change the flavour). The egg and cornstarch are necessary for thickening, and the butter is important for texture.
4 egg whites
3/4 teaspoon cream of tartar
1/2 cup granulated sugar
Again, a fairly standard meringue recipe. Leaving out the sugar would mean that the meringue won't brown on top. It is possible to make a meringue without sugar, but difficult to do, and it will unfortunately taste like dried out egg whites.
If you want the lemon pie taste but fewer calories/fat I would recommend skipping the pie crust altogether and making lemon custards. You can still top with the meringue if you want, baking in a pie pan or in individual ramekins.
At some level I find you have to accept that desserts tend to contain a lot of fat and sugar, and you just have to eat smaller portions of them to be healthy. There are options for versions with artificial sweeteners, or substituting apple sauce for oil and the like, but these won't work for all recipes, and are hit and miss with taste.