I can tell you the immediate outcome regardless of who starts the war. Defeat of Nepal. This has nothing do with the valour or fighting spirit of Nepalese people.
- India is a huge country with vast resources at its disposal. The hinterland that backs India is enormous and therefore each day the war stretched Nepal would be inching closer and closer to defeat.
- Indian military is a much more modernised force than Nepal army with combat experience on a wide variety of terrains. Nepalese army simply lacks this. This I say both about the airforce and the army.
- Nepal is a landlocked country with its only real outlet to the outside world being India. China is ofcourse there but transportation facilities along the Chinese border are yet to catch up. Therefore India could simply cut off supplies to Nepal. It would lead a financial collapse of Nepal besides starvation and famine.
There are a hundred more reasons why Nepal would fail. Modern wars are not a match up of bravery and valour alone. It would be a technological war and Nepal would not hold a chance.
I can also tell you that such a war would never happen (atleast not a full fledged war. Skirmishes might happen, but then that won’t escalate. We already have some experiences with such small scale conflicts)
- India wouldn’t want to risk its position in the world as that of a responsible nation and a leading power in South Asia. It would be a diplomatic headache to undo the damage such a conflict, if initiated from Indian side would cause.
- Nepal despite all the hue and cry over India’s Big Brother attitude receives a lot of aid and support from India in a lot of spheres. Not to mention the hundreds of Nepalis working in India (likewise Indians in Nepal).
- For India it can cause a problem in its international image and make a case against its inclusion in the UNSC. It would be like USA attacking New Zealand.
- India can use other means to gain what it wants in Nepal, both covertly and overtly, if it wants to. Nepalese politics is so faction ridden that you can always find someone who would agree with Indian view of things, given that Indian view is not harmful to Nepal. For example, Madhesis would support Indian view if India were to say that they should have their own federal unit in the country.
So the simple answer to your question is that unless the leader of either country is really stupid and doesn’t listen to expert opinion rendered, such a war has Zero chance of occuring.