Cooking the masala in the same pan as the fish is fried with amp up the flavor a lot. But you can cook them separately and add the remaining fried oil into the masala as well.
The tamarind is optional, but I find it adding so much flavor to the masala.
If you find the masala too spicy, add a couple of tablespoons of coconut milk at the end to settle it down.
Instead of using the chapathi tawa, you can bake the parcels! Rub a little coconut oil on a baking tray, layer them and bake them in a preheated 180 degrees oven for 10-12 minutes.
You can use any meaty fish instead of karimeen - king fish, pomfret, hamour works wonderful!