Carrageenan, a seaweed extract, is used frequently in vegan cheese and other vegan and vegetarian preparations as a thickening agent. However, I have never seen it recommended as a way to add "body" and texture to a vegan stock, as a replacement for the collagen in meat stocks.  Particularly, I'd like to experiment with it for vegetable stocks intended to be thickened into a sauce. Is this because it works poorly for that purpose, or just because few cooks have tried it? Or some other reason? 
In my experience, a combination of acid (lemon juice/tomatoes/tamarind paste) and heat(temperature) is used to cut the taste of raw onions. You will need to continue frying the raw onion and masala mixture until it separates from the oil, which is a good indicator of the doneness of the onions as well as the spices. If you follow this route of cooking the onions after pureeing them, you would also not want to roast the spices before grinding them, and instead fry them all together at this stage. 