Hah! I've read so many comments on this forum about Nepal and there seems to be ZERO perspectives from anyone from Nepal. Let me fill that void.
Firstly, Nepal became secular in 2008 after the monarchy was deposed. It opened up the public space for groups that have traditionally been left to the periphery of the Nepalese state. We have an overwhelming number of Hindus (as a % of the population, more than in India) but our identity is strongly tied to the state aka Nepal. Talk to any Nepalese person and you'll get a sense of how much we love our country (not necessarily the government, as in many cases throughout South Asia).
Secondly, even if the BJP wins the elections in India, it is highly unlikely that Nepal will revert back to a Hindu state. The very protector of the Hindu identity of Nepal was the monarchy and with its abolishment, Nepal's Hindu identity was also transformed into a secular one. Although people profess to the Hindu faith in overwhelming numbers, religion is yet to be politicized in Nepal (and hopefully, it never will. We've seen what has happened with our neighbors). Thus, all this mumbo jumbo being spewed by the ultra nationalist Hindu right wingers in India do not resonate with Nepalese Hindus (most of the time; we have such people in Nepal too but they are a fringe group). The three largest parties in the Constituent Assembly are the Nepali Congress (centre right party with deep connections with India's Congress Party), the United Marxist Leninists (UML - moderate communists), and Maoists (used to be ultra left wing but they've moved closer to the center). The Rastriya Prajatantra Party (RPP) is a pro-monarchist party and it came in 4th with miniscule number of seats.
Given the reality on the ground, the fear of Nepal reverting to a Hindu state is highly unlikely. In my opinion, we are better off as a secular state as we are a very diverse country. Also, please stop it with calling for annexing Nepal to India. We're two nationalities and the Nepalese (and I'm guessing rational Indians) would like to develop stronger ties between the two states. Just because most Nepalese are Hindus doesn't automatically mean we want to be Indians, just like being Muslim doesn't make one a Saudi. We became a state in 1769 and our identity is deeply entrenched.