Preheat oven to 350°. Make cakes: Butter two jelly roll pans* (10- by 15-in. rimmed baking pans) and line with parchment paper. Butter paper, then dust inside of each pan with about 2 tsp. flour. In a large bowl, beat egg whites on medium-high speed with a mixer using whisk attachment until they hold soft, distinct peaks, 2 to 3 minutes. Transfer to a medium bowl and set aside.
In the large bowl with mixer on medium-high speed, beat egg yolks, gradually adding granulated sugar, until thickened, about 5 minutes. Beat in vanilla until incorporated, then add remaining 1/2 cup flour and beat to blend. Gently but thoroughly fold in egg whites.
Divide batter between prepared pans; using a small offset spatula, gently spread batter smooth. Bake, switching pan positions halfway through baking, until cakes are golden brown and centers spring back a bit when lightly pressed, 15 to 17 minutes.
Let cakes cool 3 to 4 minutes. Run a thin knife between cakes and sides of pan. Set 2 smooth kitchen towels on a work surface and dust with powdered sugar. Turn out each cake onto a towel and peel off parchment paper. Let cakes cool completely on towels.
Meanwhile, make filling: In a clean medium bowl, beat cream, vanilla, and powdered sugar with a mixer on medium-low speed until thick, 5 to 7 minutes. Fold in chopped almonds.
Stir prickly pear jelly to soften. Spread a generous 1/2 cup on each cake, using a small offset spatula and leaving a 1-in. border along 1 long edge. Drop whipped cream by spoonfuls over jelly and spread evenly. Working with 1 cake at a time, lift towel on a long side and roll each into a smooth, compact log.
Make saguaro “stem”: Cut 4 strips of parchment, each 2 to 3 in. wide, with 2 strips a little longer than cake and 2 a little wider. (These will keep serving board clean while you frost cake.) In center of a large, flat board at least 16 in. by 20 in, arrange parchment strips in a rectangle a bit wider than cake. Lifting carefully, set 1 cake log seam side down on parchment so it overlaps it somewhat.
Make saguaro “arms”: Cut 4 strips of parchment, each 3 in. wide and about 16 in. long. Arrange 2 parchment strips on either side of saguaro stem to go under edges of arm pieces while frosting, before they are placed against stem. Slice remaining cake in half crosswise to create 2 smaller rolls.
Cut 1 smaller roll lengthwise diagonally to create 2 triangles. Place triangles on 1 set of parchment strips, one triangle seam side up and one seam side down.
Cut remaining smaller roll crosswise to form 2 round slices, 1 about twice as long as the other. Place round rolls on second set of parchment strips, both seam side down.
Make frosting: Wipe mixer bowl clean. Using paddle attachment, beat cream cheese and butter on medium speed until well blended, then mix in lemon juice and zest. Add powdered sugar and beat until smooth. Mix in 1/16 tsp. leaf-green food coloring and mix well. Wearing disposable gloves if you want to avoid staining your hands, dip a toothpick into tube of black food coloring and add 1 large smear to frosting; mix until fully incorporated. With another toothpick, add 3 smears golden yellow food coloring and mix until fully incorporated and frosting is sage green.
Press top end of saguaro stem into a slightly rounded shape. Using a small spatula, spread about half of frosting over stem to cover, but leave open side of bottom end unfrosted so you can see the filling.
Generously frost triangles for saguaro arms, carefully spreading over open sides to cover filling. Frost round cake rolls, but leave filling sides unfrosted. Reserve remaining frosting for final touches.
With tines of a fork, make long, deep vertical stripes the full length of saguaro stem, starting at rounded top; wipe fork clean and repeat to cover entire stem. (If frosting is too soft to hold a shape, chill just long enough so that it can, 15 to 20 minutes.)
Assemble cake: Carefully remove parchment from under saguaro stem and arm pieces, lifting them with a thin spatula. With a wider thin spatula (like a metal pancake spatula), carefully lift each frosted cake triangle and attach one to either side of stem, setting its short side against stem with horizontal side facing up. For a natural, “staggered” look, set 1 triangle about 5 in. below rounded top and the other about 8 in. Set round cake rolls on top of cake triangles so filling side is flush with triangles. Use reserved frosting to frost over gaps between triangles and rounds.
With tines of fork, make long deep stripes over each arm as you did the stem, starting where arms attach to stem.
Pour a little gold pearl dust at a time into a fine strainer. Gently tap strainer over cake to create some dappled “sunlight” across saguaro. Using a small amount of frosting for glue, place 2 or 3 Meringue Saguaro Flowers on end of each arm.