Coat a Dutch oven with cooking spray; place over medium-high heat until hot. Add onion, celery, and garlic; saute until tender. Add chicken and next 5 ingredients. Bring to a boil; cover, reduce heat, and simmer 15 minutes. Gradually add 1 cup potato flakes, stirring constantly. Bring to a boil.
Combine water and next 4 ingredients in a medium saucepan; bring to a boil. Add remaining 1 1/4 cups potato flakes, egg substitute, and flour, stirring well. Remove from heat. Drop dough by rounded tablespoonfuls into boiling chicken mixture. Cook, uncovered, over medium heat 10 minutes or until potato dumplings are firm. Remove and discard bay leaf.