Is there any way to increase the revolt risk for a time for the Incas?
No.
At least through the 1500s, to simulate the actual struggle to finally conquer them. And possibly include more than their home province if they revolt.
This, we can do easily. Hartmann?
Also, would adding a moderate Incan army on the border provinces where the Europeans attack be actually useful for their defense?
Perhaps... We would have to make sure that the Incas could afford the upkeep though.