Combine the gelatin and hot water in a small bowl and stir until fully dissolved; let cool. Combine the chopped strawberries, 1/3 cup granulated sugar and 1 1/2 tablespoons lemon juice in a large bowl. Let stand, stirring occasionally, until the strawberries are juicy, about 15 minutes. Stir the dissolved gelatin into the strawberry mixture.
Meanwhile, beat the mascarpone, cream cheese and confectioners' sugar in a large bowl with a mixer on medium speed until fluffy, about 2 minutes. Add the vanilla and lemon zest and beat 1 more minute; set aside.
Line a 9-by-5-inch loaf pan with plastic wrap. Pulse the pretzels with the salt and 2 tablespoons granulated sugar in a food processor until finely crushed. Drizzle in the melted butter and pulse until well combined. Pour the mascarpone mixture into the prepared loaf pan; smooth the top. Top evenly with the pretzel mixture, then top with the strawberry-gelatin mixture in an even layer. Cover with plastic wrap and refrigerate until set, about 6 hours.
Make the strawberry sauce: Puree the whole strawberries with the remaining 1/2 tablespoon granulated sugar and the remaining lemon juice in a blender until smooth.
When ready to serve, invert the terrine onto a platter. Top with the strawberry sauce.