what would happen, if there would be a war between two countries (let's say india and pakistan) and pakistan would release nuclear weapons either:
-on indian troops on their own (pakistani) soil
-an indian city
- - - - - - - - - - - - - -

oh, and the scenario in the beginning is:
pakistan attacks india which defeats them and pushes their forces into pakistan (south of islamabad, for instance, where there are not any cities, so that the B option is plausible)

I assume option A is nuking Indian troops in Pakistan, and option B is nuking Indian civilians/cities, though you seem to be mixing them up so I may be misinterpreting what you are referring to. Pakistan attacks is assumed to include actions like a repeat of the 2008 Mumbai attack.

Also, Islamabad is only the 10th largest city in Pakistan, and the 6th largest in the Punjab Province. The area south of Islamabad has a number of those cities. But let’s assume that does not stop Pakistan from using nukes in option A.

What would be an immediate international response and what would be a more long-term one (esp. to the first case, which is basically a defensive measure to an invasion)? and let's say international pressure would prevent india from retalliation and brought the war to a temporary halt.

if the pakistan would get nuked (from the usa - i've seen several posts here about such action in an event like this), what would be the legal justification for it (because we can is not an answer). that question applies to both first and the second option separately.

let's say that the nato deployed peace keepers and india and pakistan didn't want to upset them, finished the war and drew new borders at the position of the caesefire.

would there be sanctions (for both countries even?)? maybe nuclear disarmament (of both countries?) forced by the international community (what about if india used their weapons too)?

history has shown us that sanctions don't work as advertised and i've seen a youtube video of an US AF general telling the same about iran (don't have the link, will post it shortly). i suspect that pakistan would lean against china even more, but i don't know about india (outsourcing and whatnot).

some different measures (long term)? just leave them be so that they kill each other if they want?

In the event of Pakistan using nuclear weapons India’s response will most likely be immediate and predetermined, and be executed long before any communication from the US or UN could arrive.

 

As for the USA nuking Pakistan, unless a Pakistani nuke goes off in a 3rd country, due to either a direct military action or terrorist attack, the odds of that virtually indistinguishable from zero. The diplomatic fallout from such a unilateral intervention would be immediate, sustained, and massive.  If Pakistan nukes anyone but India all bets are off.

what would be the long term consequences in A and B scenario?)

Consequences of scenario A are difficult to guess and depend heavily on events leading up to the conflict, but India will certainly retaliate with one or more preselected strikes of their own. At that point it may be possible for Pakistan to back off and de-escalate things. But it is far more likely that events just proceed directly from scenario A to scenario B.

 

In scenario B the results will be the destruction of Pakistan.   Pakistan has more bombs, but India has many times more people and cities, and a much larger army that will be bent on revenge.   After the initial spasm of nuclear strikes, the big question will be whether Pakistan will offer unconditional surrender or India use it as an excuse for genocide against the tribal groups that supply the vast majority of the religious fanatics , or extends it to the surviving Pakistani population in general.  It is probable that combined casualties will be well in excess of 100 million.