I thought apart from Assam, the rest of sates in NE did not have the historical/cultural/ethnically bond with mainland India, and people on the land were largely tribal people who did not have the modern day "nation-state" concept at the time. They became parts of Indian Union was totally the legacy of British Raj . The momentum of British colonial rule took them well into after India's independence. Hence the calm and lack of resistance. For them, under either Indian or Chinese rule could meant the same thing.
Yes to the bold part.
Yes, and no.
Manipur and Tripura were different; they had got transmogrified. The phrase that fits but I don't want to use is 'Sanskritised'. What you are saying is more true of Nagaland and Mizoram, and less true of Meghalaya.
Incidentally, when considering Nagaland, and parts of Manipur, it must be borne in mind that colonial conquests created an Africa-like situation, where some tribes were trifurcated, among two states governed by the British under the rubric India, and one geographical location within Burma. It would take too long to go into the details, but the Nagas, for instance, are split among Nagaland, Manipur and Burma.
Actually, it is too tempting to avoid a small detour: the tribe in question is the Tangkhul Naga tribe. They are thought to have originated around twenty centuries ago in China, and migrated in stages across Burma to the location of the Burma-Manipur boundary. This boundary was a creation of the British and the Burmese, and the Tangkhul had no idea that they were being partitioned!
Because they are considered a Naga tribe, the Nagas want them all in one state of Nagaland, all including the Tangkhul in Manipur and those in Burma. The Tangkhul themselves are quite happy to be Tangkhul, and don't especially yearn for absorption within the greater Naga masses. Tripura is far more 'settled' in character (an unlovely word, implying that others are less 'settled', and I cringe at having had to use it).
That is the problem with history, there is not prefect way to heal the wound left by it expect time itself. People often compare Japan and Germany after WW II, however one thing they don't realize is that Germany after Hitler was a totally different country where as in case of Japan it is not.
Back to the matter of 62's war, I think that everyone was at fault there as I stated years before, and there is no definite answer of who had more to blame than the other. Even there was a time machine to take all of us back in time to witness everything and every decisions that were made back then, I don't think we are able to make judgement of whether they are right or wrong simply we are not living in their time (I am not sure about your case though)*.
*I acknowledge being a troglodyte. Give me a little time; very soon now, I shall emerge from my cave and re-enter the human race at least in the eighteenth century. It may take a little while to complete the remaining three hundred years.
Personally, I think it was an egregious mistake by Nehru, and his complete lack of experience in diplomacy and external relations. He tried to apply his Fabian socialist background, and some of the ideals of the freedom struggle, meaning Gandhi's thoughts cleaned up for international consumption and couched in graceful terms, to relations between India and her neighbours, and really made a mess. I hate it when he is criticised in the vulgar, gutter language that some really uncouth young Indians use, but that is because they seem to overlook the cementing force that he was in the early years of the republic.