You have tea, cotton, and a couple of spice plantations in India. Yes upgrading the plantations helps.

I don’t think there is a price drop you can use to your advantage. Getting rid of others trading the goods is the better method. At least reducing their trade ships.

The trade rout thing is unavoidable. It only gets worse if you go to war over it. The God of the game AI hates the player. Anything that will hurt the player is fair game. In ETW you just have to live with it.