This is not a technical problem and should not be automated away.
Rely on trustworthy third parties. Universal utilities like Google should have retail outlets which are adapted to local conditions and can exercise educated judgement. In some countries, the police might certify the identity of the individual, and then Google could trust that certification. In another place, it might be some combination of the Red Cross and a public hospital. Obviously some identifications will be easier and others harder - if a person in New York claims they are the owner of an account based in Spain, the employee should be suspicious and require a higher burden of proof (and the reactivation might be logistically more difficult).
> The other thing is, we want at the same time Gmail to be unhackable against best hackers and state sponsored adversaries for the billions of users, including high profile dissidents, journalists, and senators who will inevitably have accounts;
I'm not really convinced high profile dissidents, journalists and senators (why senators?) should be trusting Gmail to protect them from state sponsored adversaries. Google generally wants to do business in territories controlled by states which means they have to follow laws and will sometimes be subject to intimidation; but they have no intrinsic motivation to be unhackable.
