Combine tomatoes, celery, jalapeno, horseradish, and 2 tablespoons lemon juice in the bowl of a food processor and pulse until smooth.
Line a medium nonreactive bowl with a double layer of dampened
cheesecloth. Pour pureed tomato mixture into lined bowl. Gather together ends of cheesecloth, creating a sack for the mixture. Tie with kitchen twine; hang from a cabinet door or knob above the counter, keeping bowl underneath to collect juices, and leaving about 4 inches above bottom of bowl. Let stand, undisturbed, at least 2 hours or preferably up to 12 hours. You should have about 2 cups collected juice. If not, gently squeeze sack to release more juice. Season juice with salt and black pepper; discard solids.
Ladle 1/2 cup collected juice into a medium bowl, and sprinkle with gelatin. Stir to combine; let soften, about 5 minutes. Prepare an ice bath; set aside.
Pour remaining 1 1/2 cups collected juice into a small saucepan, and bring to a boil. Immediately remove from heat, and pour over softened gelatin. Stir until gelatin dissolves, then place bowl in ice bath to cool quickly, stirring frequently; make sure gelatin does not begin to set.
Pour 1 tablespoon gelatin mixture into each of 10 serving glasses. Add 2 cherry tomato halves; let stand just until liquid starts to gel, about 20 minutes. Repeat with another tablespoon gelatin mixture and 2 more tomato halves. Cover tomatoes with another tablespoon gelatin mixture, and transfer to a refrigerator to gel completely, about 1 hour.
Place crab in a bowl with remaining 1 1/2 tablespoons lemon juice and the oil; season with salt and pepper. Toss to combine. Divide crab mixture among glasses.
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