Instead of white pumpkin /Ash gourd can use even veggies like ladies finger.
I add butter milk only after the gravy is completely warm in this way the mor kulambu will be creamy and prevents curdling.
For buttermilk you can make it from the curd you are having and butter milk need not be too thin should be thick.
I always fry the spices in little oil before grinding for nice aroma and flavour.
If you are adding butter milk to hot kulambu definitely the gravy will curdle.