Generally, most people(including pakis) assume that as soon as India weakens pakis will pounce on India to claim their pound of flesh, just like 1965.
But, it seems to me that a new factor has risen in the recent times that is ignored in such calculations. The new factor is the local jihadis of pakistan. Now, these local jihadi warlords are nipping at the heels of the PA. Just as PA are waiting for India to weaken, jihadi warlords of pakistan are waiting for PA to weaken. Just as India has to take into consideration that it may have to fight a two front war with PA and PLA simultaneously(or war with one followed by the other). Similarly, PA may have to take into account that it may have to fight a two front war with India and local jihadi warlords(or war with one followed by the other).
I know that frequently, in India, one is told that all the warring groups of pakistan will unite to fight India. It may be true. But, it may also be a PA propaganda, or a convenient posturing by the various factions in pakistan. Even if one assumes that all the factions of pakistan will unite to fight India, what happens after the war? This is an important point. So, various factions of pakistan may take interesting positions with a view at post-war scenario. The post-war situation must be taken into consideration by the PA also(what if PA weakens too much without any clear victory? Jihadis can use this situation to dislodge the PA and claim the throne). Of course, there is every chance that PA will opt for a tactical brilliance ignoring such calculations. Or PA may prefer a self-preservation. It would be interesting as to how Vivek Saar will deal with this.
Actually, it seems to me that any such war can have a chain reaction, with a potential to even start a global war.
If India weakens too much, then pakis enter the picture. Even naxals, indian mujahideens, khalistanis, or dravida-nationists and a host of other such entities can increase their activities. Even BD can start making moves egged on by the others.
Similarly, if the chinese weaken too much, it also faces disintegration along with loss of lots of territory and even potential aggression from Russia, Japan, and even Korea; not to mention US. Apart from that, CPC faces a potential revolution.
In a certain interesting post-war scenario, China and India may unite with each other in a diplomatic or even strategic alliance to ward-off any threats. Lets say, India and China are really weakened after the war, then they may find that many hostile forces are waiting to rip them off. Ironically, they may find that the only ally in such a scenario is each other. In fact, it seems to me that enmity can, many a time, lead to respect and alliance(when interests are common). Many enmities end up with alliances, in history.
This raises a question: at what point will China or India start considering to stop the war. How much is too much for India and China?
The main problem in India and China relations is that China does not respect India. China does not see India as its equal, so there is no question of alliance between un-equals. I don't think it is completely china's fault. India's behaviour is such that it does not evoke respect, leave alone fear. This alone would not have been much of a problem. The problem is that India has great potential. This makes India a threat to China. To give an analogy, India's behaviour is like a mighty elephant acting like a pet-dog. So, China does not have any respect for India and yet it fears the potential of India. This is the whole problem.
If there is war between India and China, where China is given a bloody nose(or much more), then China may start respecting India and look at India as not only a potential threat, but a present power that must be given its due. With such an attitude, it is entirely possible for Chinese to agree for an alliance with India. After all, nothing succeeds like success. I don't mean to say that India and China will become great buddies(of course, even that may happen)... India, it seems to me, always tries for alliance/peace/truce through conciliation. But, it seems to backfire because most of the times it is seen as weakness by the other party. I am not saying friendly noise is to abjured, but conciliation is the face of extreme provocation may not lead to friendship. Most of the times, it would be interpreted as weakness. Maybe, India should try some tit-for-tat stuff and it may convince other party to respect India and hence agree to an alliance/peace/truce.
Actually, the above assessment applies to not only India-China relations but many relations that India is involved in.
In the present scenario, when the pakis see that Indians are giving chinese a very hard time, will the pakis jump in? There is greater likelihood of pakis jumping in, if the chinese are already winning. But, if the chinese themselves are having a hard time and want the pakis to jump in to take fire, will pakis do that? It is more probable that pakis will find some silly pretext to not do that. They may say they are mobilizing, and keep delaying the mobilizing. Pakis may even say that they don't have funds for some urgently needed equipment to fight a war and ask for more and more aid from the chinese. may be pakis will make lot of bluster accompanied with incidents like breaking the cease-fire at LOC, threatening to open a new front, but just stop short of doing it. Or Pakis may jump in, albeit reluctantly, and retreat at slightest pretext.
About US and Russia's reaction: I am not sure. I think they will not like Indian aggression. I think they prefer status quo with a weakened China and India.