I don't think people consider nuclear war an a high probability event, but when long term enemies have the weapons, it's not an unreasonable speculation, IMHO.
Effects: A lot of destruction on the subcontinent, of course.
Long term: Whoever is perceived as responsible for the use of nuclear weapons is an international pariah. That's the case even more if cities were targeted.
When the dust settles, there will likely be a widespread revulsion focused on nuclear weapons. I don't think that it would result in total nuclear disarmament, but might result in more reduction of existing stockpiles. I'd say that it's almost certain that nuclear proliferation will not be tolerated.
The politics involving Isreael's possible nuclear arsenal just got interesting...
...SNIP...
In 1999 Pakistan didn't have the number of nukes that they possess now the build up took place post 2005 at the request of Saudi Arabia as there is an agreement to "sell" nuclear weapons to Saudis in the event Iran goes nuclear
So the war if it does go nuclear it would be limited nuclear exchange there will be loss of life and political instability but certainly not nuclear winter besides nuclear winter is a myth created by the KGB in the 1980s in response to US military build up .
Could you please provide some sources on the Indian nuclear build up being at Saudia Arabia's request, and your claim about nuclear winter being a KGB inspired myth?