Some raw thoughts based on above genuine ideas and comments...
On receiving valid reviews.
Have a JED rating component in the j! back end mandatory to file a review. Admins can only review from within the back end of their site.
Sometimes an extension breaks a site. It needs to be uninstalled immediately. In order to still be able to review, have a list of used third party extensions used. From that it will automatically create "proof" it exists/existed and has been installed (before).
(Allow admins to purge that list, once purged, no reviews can be made.)
(Include a "showcase" check box for those that want to show off their site
Have plugin like System Joomla Statistics enabled and used at least once before to check against the ID.
ID should be accompanied by a registered reviewer. When you register as user, you receive an api key to use in the plugin. Review is checked against extension with version number, site ID and reviewer api key.
When as a reviewer already commented once there is no extra review, the JED will then show as how many times reviewer installed it. Adding extra info to the review. (and if show case checked, shows the site where it is used, should be unique)
In short it means a reviewer should really have installed it before any review could have been made. It also proves the reviewer actually has experience with the extension and how many times.
On Developer proof.
One of the comments above was on version number fraud by developers... when a new version is being made, take some into account.. a version number is like x.y.z. as mentioned, the numbers should be mandatory. On version numbers actions can be taken.
x being the major release (change, purge reviews as the previous has ended, reviews are no longer valid)
y being minor release (not changed within a year, lock reviewing)
z being a bugfix/security release (allow reviews to be kept)
Purge anything that is no longer compatible with active supported Joomla! versions. (already been done I believe)
On each change, developer is to publish a release note / change log.
That is something that can be enforced on the JED. Something that actually proves a change has been made.