Is it advisable to forgo admission offer from IIM Calcutta and prepare for UPSC? How risky is it? And is the reward worth the risk?

Forgoing an admission offer from Calcutta and preparing for UPSC is a risky choice. Whether this is a good decision depends on your goals and how soon you are willing to achieve them. If you are falling short of time, then you can devote the entire 2 years to UPSC preparation. 

But if you prefer quality over quantity then my suggestion would be to pursue MBA along with it. Surely the first year will be challenging. You might face serious issues managing both the tasks at hand. But trust me the results will be amazing. Pursuing an MBA will change your perspectives regarding the issues around the world. You won’t just study for UPSC you will learn from it. Moreover, you can select management as your elective in UPSC. Management and administrative services are correlated. 

If you have less time in hand forgo the admission offer and prepare for UPSC. But if you want to crack a qualitative UPSC pursue MBA along with it.