Should include extensions that have had a change in developer, e.g. sold.

Is it possible to tell that if the sale included the whole google account?

I would imagine many developers upload extensions using their personal Google accounts and would not want to part ways with their account.

so this would become a loop hole - once sold (secretly/discreetly), you claim to want to move the email to a non-personal account (but it's actually controlled by the buyer). Noone would be able to tell the difference between that, and the author actually wanting to move away from using their personal account for the extension.

Require change of ownership as a condition to notify Google. You can get around it but Google can also ban you if and when they find out. And it only costs the time to have Legal add the terms to the TOS.

This is a legal/contract problem, not technical.

Wouldn't it be better to notify users if a long unupdated extension suddenly updated?

That's trivial to work around. It would lead to wasted resources where developers push out no-op "updates" to keep from being stale, and in the case of a sale a condition of the sale would be that the original developer pushes out some benign updates prior to the sale closing (or the new owner would push a few benign updates until people stopped looking).

In other words it would cause more work for good developers and extension reviewers and grant no additional safety.

Very good adversarial thinking.

i think adversarial thinking is what more people need to do when thinking up policies and/or metrics to measure/monitor something.