? 
"But do you think India will really give us those parts?"
It was never theirs. They won't give. We will snatch.
"This is true that India is capturing our land illegally but where is the place or where is the authority which will take action against India?"
"The People" is the authority. No one is seeking any action against India per se. What we are asking is restoration of our land. There are stages of protest: Nepal, India, China, UN. If it doesn't help, there are other methods at our disposal. I don't want to be explicit on that.
"If we are struggling to protect the lands which are under our own control right now, will we be able to get those parts which have been used by India for several years?"
True, but it is equally true that the root cause of our problem to control our existing land can be tackled by having our lost land.
"And if this issue really goes up, what will be the reaction of India on it?"
How do you think an expansionist will react? It will cry and bang and bully, but that is NOT going to stop our movement.
"But this is our necessity to keep good relationship with India."
True. But it is about mutual respect and co-existence. Big fish eats small fish but its for us to learn to swim out of the shadow of the Big fish. A small fish (a motivated one) can eat a big fish -it’s just a matter of strategy and determination.