One way to keep the invaders out is to hire mercenaries. One problem with mercenaries is that it's difficult to sustain their loyalty over a long period of time. A way to counter that is to marry off the mercenaries to local girls. The recruiting poster would be "Come to Harappa, join the army and take a wife!"
Of course there's no evidence of this.
The city was besieged, overrun and the city dwellers massacred. Happened often enough in recorded history. More than just plausible here.
The other information gleaned may or may not have anything to do with it. If somehow evidence indicates that it does, then your scenario comes right to the forefront.
City-states rising, becoming wealthy and powerful, sliding into decadence, then becoming unable or unwilling to withstand an invasion and falling seems to be predictably cyclical to the point of inevitability.
Tribes further away would be busy taking in Harrapa's children ~ more or less like a large daycare operation.
So, who else did this for nearly 800 years? That place is well known as the Republic of Venice!
In Christopher Columbus time it was common for French nobles to farm their children out to Italians living in small villages away from the (for then) large cities and towns. A downside was if your real folks back home died in the palace, or in a local war, then you'd grow up as an Italian townsman.
That seems to be what happened to Chris!
Join and become a citizen!