Sauté oil, butter, 2 celery, 2 scallions, and some parsley and thyme leaves with onion in a saucepan over medium-high heat for 4 minutes.
Add shrimp heads and shells, cook for 2–3 minutes.
Add 6 cups water and bring to a boil. Reduce heat to low and simmer 25 minutes, skimming foam occasionally.
Strain the stock into a clean pot; discard solids.
In the same pot, sauté remaining celery, garlic, pimento, scallion, thyme sprigs, and black pepper in 1 tablespoon oil for 4 minutes.
Add carrot, pumpkin, sweet potato, and green banana slices; pour in the strained stock and bring to a boil.
Season with salt, Caribbean green seasoning, and add the whole scotch bonnet pepper; simmer 20–25 minutes until vegetables are tender.
(Optional) Stir in noodles during the last 3 minutes.
Off heat, gently add seasoned shrimp to the broth and cover for 8–10 minutes until shrimp are cooked through.
Taste and adjust salt. Serve hot.