Given the premise I don't see how war could possibly have been avoided no matter how difficult it was bound to be. Congress might actually declare war the old fashioned way.
Yes, the war with Taliban is still on the table. The war with Pakistan is occurring because they're protecting the Taliban and are believed to have supported the 9/11 attacks.
Pakistan had roughly 140-50 million in 2001.
The war would likely open with some kind of first strike on Pakistan's nuclear capabilities. I don't know whether that would have necessitated the use of nuclear weapons, probably not but it would likely be considered.
There would be a huge air campaign, one that would likely last for months.
Even using low end Rumsfeldian calculations the US would need at least 1.2 million men to actually invade Pakistan. (170k for Iraq, multiplied by 7)
A complete call up of the Reserve and the National Guard could mobilize an invasion force of perhaps 400,000 (complete guess on my part, feel free to set me straight) in six months. They land in the south while a million Indians come in from the east.