Preheat oven to 350°F. Coat a 9-by-5-inch (or similar-size) loaf pan with cooking spray; dust with flour and tap out any excess.
Sift whole-wheat flour, all-purpose flour, baking powder and salt together into a medium bowl.
Finely grate 2 tablespoons zest from 2 of the lemons (see Tips); set the lemons aside. Beat 3/4 cup sugar, cream cheese, butter and the zest in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 2 minutes. Beat in egg whites, one at a time, beating well after each addition. Add the egg and beat well. Reduce speed to medium and beat in milk; the mixture will look curdled.
Reduce the mixer speed to low and add the flour mixture in 2 batches, beating just until combined and scraping down the sides of the bowl as necessary. Transfer the batter to the prepared pan and smooth the top with a rubber spatula.
Bake the cake until a wooden skewer inserted in the center comes out clean, 45 to 50 minutes. Cool in the pan on a wire rack for 15 minutes. Run a knife around the sides and turn the cake out onto the rack.
While the cake is cooling, squeeze 5 tablespoons juice from the zested lemons. Trim the ends off the remaining whole lemon and very thinly slice; discard any seeds. Heat the lemon juice and remaining 1/2 cup sugar in a small nonstick skillet over medium heat, stirring until the sugar is dissolved. Add the lemon slices and cook, stirring frequently, until softened, about 5 minutes. Reduce heat to maintain a simmer. Use a fork to transfer the lemon slices to a bowl. Continue simmering the syrup until slightly thickened and beginning to turn golden yellow, 2 to 4 minutes.
Set the rack over a rimmed baking sheet. Poke holes all over the top of the warm cake with a wooden skewer, 1 1/2 to 2 inches deep. Spoon the glaze over the cake, poking more holes if the glaze does not sink in. Arrange the lemon slices on top. Let the cake cool completely before slicing.