Being a retired accountant, I feel that your theory has merit, EM. Moreover, I've suspected that this has been going on for a long time. But until we can get a hold of the Watchtower's financial statements, it must remain just a theory. Perhaps that is why the Watchtower refuses to release their financials to the public. If the Watchtower's financial dealings were honest and above board, they would have no reason to withold them from the public.
While it's not illegal for the Watchtower to pay a JW owned company twice as much for supplies as it would to a non-JW company, if this became public knowledge, the rank and file JWs might think twice about paying $20,000 for the one day use of a building that the R&R JWs built and paid for.