Pop the popcorn in a popcorn popper according to the manufacturer's instructions. While still hot, sprinkle with the white Cheddar popcorn seasoning to taste. Let cool completely. (You should have about 12 cups.)
Whisk the mayonnaise, sour cream, cider vinegar, sugar and Dijon together in a large serving bowl. Stir in the shallot. Season with salt and pepper (remembering that the popcorn will also be salty).
Add the snap peas, carrot and chopped celery and toss to coat. Add the popcorn and toss to coat the kernels in the dressing. Pile the watercress on top and sprinkle with the celery leaves. Serve immediately.