I don't agree with this part. At the time of the partition J&K were part of the princely states which is why they weren't incorporated into either dominion (India or Pakistan).
The ruler of the princely state(in this case maharaja hari Singh or whatever his name was) had the authority to decide on behalf of the people of j&k. Hence, I don't agree with with ur point that J&K should've joined Pak instead of India.
The Kashmir problem has raised due to Pakistan's greed and misadventures. Had they not forcefully tried to capture Kashmir and let Kashmir remain an independent state, maharaja wouldn't have asked for Indian army's help and this long lasting conflict could've been entirely avoided.
It's not necessarily illigel per say. Its been stated that article 370 can be abrogated by taking the constituent assembly into confidence. However there's no assembly in J&K right now and the state is currently under presidents control.
After the new amendment, the government of J&K equals to the governor of J&K and the constituent assembly. The recommendation of constituent assembly is is required to abrogate the law and now this constituent assembly equals to the state legislatior. This means, in order to abrogate the article 370, president just needs recommendation from the state governor. Nothing else.
Don't really know how it's a trilateral issue between BD, Ind and Pakistan. How Bangladesh comes into the picture?
Fact is, this stupid law aka article 370 and subsequent 35A has failed to bring any fruitful change in Kashmir in last 70 years. As a matter of fact it has made it difficult to integrate Kashmiris. These spacial status has further alienated Kashmiris and influenced them to get involved in a sorts of illegal activities.