Hello. I am not sure about Aryans leaving Harrappans alone.
Aryans conquered this area, and imposed most of their culture and values over Harrappans. The conquered people eventually had to give up their culture etc. And now we have no signs of Harrappan culture left in Pakistan.
However, Aryans also got effected by many elements of Harrappan life, such as yoga, meditation, karma, etc.
Buddha’s time may have been the time when Harrappan culture and religion was still known to people. Buddha picked up on that, and created his religion.
His religion had become quite popular in this area for long time. Had Buddha not revived Harrappan religion, it would have been lost to the world just like several other attributes of this lost civilization.
Not sure about that. Will look for it.