Suppose there is a piece of land between your house and your neighbour. Both of you claim that piece of land as your own. So you go to a court of law to settle your case. However, it turns out, that the piece of land is within your boundary wall. While the case is being fiercely fought in the court, one fine day you notice that, that piece of land has termite infestation that needs to be dealt with. Now, tell me, can your neighbour tell you how to deal with that infestation, let alone prevent you from dealing with the infestation, on the grounds that the ownership hasn't been determined yet? Of course not.
This is basically the essence of legality of India's position - possession gives an inherent right and obligation to administer. (Recall that legal adage, 'possession is nine tenth of law'.)
Additionally, there is nothing in any UN resolution where the sovereignty of Kashmir has been questioned. On the contrary, the resolutions, unequivocally declare that it is Pakistan which is required to withdraw from the part of Kashmir that it now occupies. Not India. This is an implicit acceptance that it is Pakistan which is illegally occupying Kashmir. Not India. By virtue of IoA, the administration of Kashmir had been handed over to India in a full proof legal procedure, making administration of Kashmir prerogative to GoI.
That's how Kashmir is India's 'internal' issue.
The rest is the usual Pakistani drivel.