Few things in my opinion, Nepal will never be taken by India or USA, namely because of China being in close proximity and China will never take over Nepal either because of India and USA influence. Having said that why would anyone want to take over Nepal? what is the ROI that will justify waging a war to takeover Nepal? India will never annex Nepal because they know that is not going to be fruitful for them, they have learned their lesson from sikkim and mizoram/manipur headache.  They are already influencing Nepal's policies by pressuring the ruling party, whether it is maoist, congress, monarchs or otherwise by dictating the policies that favor them so why even bother? ultimately our geographical location forces us to comply with them and there is very little we can do and they very well know this.

In that regards India has already taken over Nepal, modern day occupation of a foreign country does not involve forceful takeover, rather it involves creation of policies that favors one's nation as much as possible without having to shed a single drop of blood.