How do you make a third-party pedestrian to sign the waiver?
Pedestrian would have the claim against the car driver surely, the car driver would be unable to blame the "autopilot", just like they can't blame cruise control today.
by bumping into them?
You don't. The waiver between the ownee and manufacturer specifies that the owner is liable. Beyond that point, insurance solves the issue. Currently, the driver is liable and is required by law to carry liability insurance, so there is not much change from the consumer perspective. From the insurer perspective, the relevent data point us how often self driving cars mess up compared to human driven ones.
This is a good point that introduces more complexity into the problem. Whether or not AI driving flourishes can be determined by more than regulations or manufacturers themselves. If insurance companies decide from their risk calculations that insuring cars with an auto-pilot is too much risk and therefore refuse to insure them, that could also kill self-driving cars.