In reality, none of us know the number or names of associated corporations the WT may operate or are associated with. Even if WT did make an annual audited financial statement available for public viewing, I suspect that would not reveal its entire financial position.
If WT assigned trusted specific elders to independently register and manage various charities with no recognizable link to itself, then funds may be donated on an ongoing basis from one charitable organization to another. After funds are donated, the donating organization can claim those funds are a charitable expense even as those funds remain within another associated charity.
I suspect WT mandated suppliers who charge substantially higher than market rate, are companies likely owned by JWs who agree to donate all premiums charged to a WT associated charity whereby the donating company will receive a tax receipt for its donation. Not only would the supplier reduce its taxable income, WT then would also indirectly receive a share of all mandated products and services purchased from those suppliers.