at least BetterSnapTool was never available for free and I believe I can see the spikes in sales when the fake reviewers did purchase (just before they reviewed). Not sure whether one can check the pricing history of an app, maybe there are some sites which collect that data?

That's a theoretical possibility, but for most of them there's no indication that they've ever been free, except for RapidClick, where some of the non-fake reviews do seem to indicate it was free at some point.

Also, this doesn't explain why all of the fake reviews are occurring during the same one month period.