Preheat oven to 350ºF. Line a 9"x2" square baking pan with parchment paper, leaving an overhang on all sides, then combine all crust ingredients right in the lined pan. Use your hands to press the mixture together until evenly moistened (it will resemble wet sand), then firmly press it down evenly into the bottom of the pan. Bake for 10 minutes, or until a shade darker and smelling fragrant, then cool on a wire rack.
Combine lemon juice and water in a small dish, and sprinkle gelatin over top to soften for a minute (it will resemble the consistency of applesauce).
Puree 2 packages (2 1/2 cups) of the raspberries in blender until smooth. Force puree through a fine-mesh sieve with a stiff spatula into a small saucepan to remove most of the seeds (if you have a good, high-powered blender, you can skip this step - mine blends the seeds completely).
Add sugar to the raspberry puree and bring to a simmer over medium-high heat. Reduce to low and stir in gelatin mixture. Cook, stirring constantly for 1 full minute, to ensure gelatin is completely dissolved. Transfer mixture to a large bowl and cool at room temperature for 30 minutes.
Whip the cream to firm peaks in a standing mixer, or with hand-held beaters. Fold 1/3 of the whipped cream into the raspberry mixture first to lighten it, then gently fold in the rest using a light hand until completely blended. Refrigerate for 2 hours until set (or longer - it'll still be fine, but the gelatin firms more over time. Best on the day it's made).
Meanwhile, make berry sauce for topping: sprinkle 1 tbsp granulated sugar over 1/2 remaining package (a heaping half-cup) of raspberries in a small bowl; let stand 15 minutes then mash with a fork.
When dessert has set, drizzle with sauce then top with remaining 1/2 package (another heaping half-cup) of raspberries and a handful of pomegranate arils, if desired. Slice into 16 squares, rinsing knife with hot water between slices for clean cuts.