if and when they invade, are they gonna bring S400 into Pakistan to to counter Nasr? Since Nasr is only for use within Pakistan itself.
Nasr is for defense (within Pakistan borders) not for offense, therefore the question of countering multiple defense shields does not arise
No, S400 even though a mobile system, is being acquired by IAF and not the army.
Indian army will use QR SAM and Akash NG for air defence of it armored columns.
But advantage of S400 is its immense range against the aircrafts. Even if just 2 systems are deployed a 100 Km inside the Indian borders, one each states of Punjab and Rajasthan, they cover 80 % of all Pakistani airspace. i.e . Any Pakistani aircraft especially the AWACS and tankers can be shot down, flying even up to 300 Km inside Pakistan's own borders.
To defend Indian cities from Pakistani strategic Nukes, India will multi layered ballistic missile shield.
S400 deployed near Pakistani borders to destroy the ballistic missiles in Boost or midcourse phase. PDV to destroy BMs in midcourse and terminal phase and AAD to intercept the BM in its terminal phase, if rest of missiles have failed to intercept it.
There is a fair chance neither QR SAM or Akash missile might not be able stop NASR armed with tactical Nuke from destroying an Indian IBG.
But Indian response to any nuke attack on Indian forces anywhere is fairly standard, that is a, massive nuclear retaliation.
So the question is will you launch a tactical nuke against an Indian IBG considering -
1. You will be nuking your own country - nuclear radiation will be your soil for hundreds of years, there will be birth defects in that regions population(your population) for generation to come. On the other hand, Indian IBG might be already near, one of your major population centers like Lahore, which is only 30 Km from Indian border.
2. India will respond with massive nuclear decapitation strike on Pakistan, you would loose a good amount of strategic, nuclear, missile and command and control infrastructure in Indian nuclear strike.
3. With your airforce out of the equation due to S400(and vast network other SAMs and IAF) you will have to rely solely on you missiles to deliver the nukes.
4. With multilayered missile defences in place India. There is more than a fair chance that any nuclear missile launched by surviving Pakistani nuclear forces will be intercepted, even MIRV missiles can be successfully intercepted in their boost or midcourse phase, before they can deploy the MIRV warhead.
5. Even if a few of Pakistani missiles are able to slip through Indian missile defences and hit Indian cities, India will respond with strategic nuclear strikes on all Pakistani population centers, therby ending Pakistan as functioning entity.
These are the questions, your nuclear command authority will have to ponder, before they decide to launch a itsy-bitsy tactical nuke on a small Indian IBG.