It seems like the round trip time for that type of operation would be pretty challenging for situations where you have, for example, six seconds to avoid crashing into a road barrier..
The most extreme type of assistance could maybe be a human drawing or modifying the vehicle's planned path (if you watch any of the Waymo example videos, you'll see the immediate path that the vehicle is planning to take). But even in this scenario, the car is still responsible for navigating that path and avoiding any obstacles.
Of course, how inconceivable that a self driving car could crash into a static barrier!
It it could prove this 100% to itself, it should be able to prove it to the general public and the authorities in a test environment..