Well thought out with one fatal flaw.
You assume that the current method most folks are using to develop and test these systems, public shadow/safety driving, is tenable. It is not. Not remotely.
It is impossible to drive the one trillion miles or spend over $300B to stumble and restumble on all the scenarios necessary to complete the effort. Many of which are accident scenarios no one will want you to run once let alone thousands of times. Also handover cannot be made safe for most complex scenarios, by any monitoring and notification system, because they cannot provide the time to regain proper situational awareness and do the right thing the right way.
Now switch 99.9% of that out and use aerospace/DoD simulation technology and systems engineering and we are good to go.
SAE Autonomous Vehicle Engineering Magazine — End Public Shadow/Safety Driving