I think collapse of Pakistan is a foregone conclusion. India cannot prevent it.
India never tried nor has any reason to prevent Pakistan from collapsing.
It is just like USSR. USSR collapsed because it could no longer compete with the West.
There is nothing like USSR vs West between Pakistan & India. Gap is much higher. We never see Pakistan competing with India on every front.
Pakistan's most policies are India centric but India doesn't respond with same.
Unlike USSR, Pakistan never tried either to compete with India. It only maintained the power to give unacceptable damage to India and threaten it by its suicidal policy.
These are psyops similar to North Korea.
Case is that power differential has become so large that even that bluff can't work anymore.
Similar dynamic is happening between Pakistan and India.
Only Pakistani fanboys promote India vs Pakistan like Germany & France & US vs Russia.
Truth is that India & Pakistan are more like Russia & Ukraine and even better like US vs North Korea in a couple of years.
WW2 Germany like moment was already there for Pakistan in 1971. Pakistan didn't learn from it.
Denuclearization of Pakistan is key to ending its role in spreading terrorism.
Terrorism won't stop till its existence. Pakistan being an Islamic Republic is enough for its role in terrorism.
This is something which will require resources of NATO in addition to India.
Simply bombing of nuclear assets and defeating Pakistan in a full scale war, doesn't even complete force if India alone, NATO isn't needed at all.
Question is why NATO would help India ever to eliminate proxy, they created at first place to keep India in check?
We cannot predict the political landscape after collapse of Pakistan. Most likely scenario is split into 4-5 parts. India may not merge Sindh with itself as you suggest. But a federation of Sindh and Baluchistan may emerge.
Pakistan has collapsed long ago. The Pakistan what Jinnah founded in 1947 has ceased to exist with fall of Muslim Nationalism in Indian Subcontinent in 1971 war.
Current one left on map is crisis state which has already been decaying slowly and made to float a while by US, UK & China.
It won't lost long.