The requirements of the conspiracy angle is really all you need to consider to discount any sort of broad payout scheme. To make it effective enough to get any sort of significant bump in the various critic's score measurements it would have to involve dozens if not a couple of hundred critics. All it would take is for one or two to be incensed at trying to be bought off to expose the whole thing. And what would that do to Disney's reputation? Not to mention the reputations of the critics or the publications/sites they work for. Disney might be able to recover but the individual critics themselves would be done. Would anyone accept a payout knowing it could end their career? Or maybe Disney having their own little Gitmo down in the basement of Sleeping Beauty's Castle to stash those who refuse is part of the scheme.
And that doesn't address why Disney is supposedly the only studio capable of pulling this off. All of the major studios are just as big or are part of conglomerates that are just as rich and influential as Disney. Do they not want good reviews of their big budget movies to? Why would so many studios play games with media embargo dates if they could just send out some intern with unmarked envelops to grease the wheels for a high RT score? Do movie reviewers not fear having access to their advanced screenings revoked?
And you would think that if Disney did have a bunch of critics on the payroll they'd have reminded them of it when they got all uppity not too long ago:
Critics Groups Won't Consider Disney Films for Awards Until L.A. Times Ban Lifted