What if India had gone to war with Pakistan over the Mumbai attacks?
M.A.D. didn't stop Pakistan from launching the attacks. China is the far bigger stumbling block here.India has one of the largest and most powerful armies , Navies and Air forces in the world , however Pakistan's nearest ally happens to be China and any attack on Pakistan will likely cause further tension along India's border with China . This one is a scenario that is just about impossible , M.A.D is as close to the reason why it could not happen as I am prepared to go .
M.A.D. didn't stop Pakistan from launching the attacks. China is the far bigger stumbling block here.
So IMO in the unlikely but possible event of China being hit with a terrorist strike by Muslim secessionists from Xinjiang around the same time India could have gone for a conventional response with the silent blessing of the PRC and Pakistan wouldn't have gone for all-out nuclear war over it.
What if India had gone to war with Pakistan over the Mumbai attacks?
They fire a few hundred/thousand shells cross each other's borders and launch dozens of airstrikes. A division or two cross the border and seize some disputed areas and fight some bloody skirmishes. After a week or two of this the USA/China/UN/Someone arranges a ceasefire.
Few thousand dead combined casualties and another Wikipedia article.