On the tax incentive to promote donating works to public domain
The issue is lost when you suggest that a tax incentive should be given to induce authors to donate works to public domain. Economic analysis (it would seem to me) would think that proposal quite a paradox.
The Government incentive already exists. Copyright in itself is an artificial (although necessary) Government interventive action to induce authors to create. If public interest advises that works should remain in public domain, just cease or limit the incentive already granted. Can you imagine a Government subsidy to plant wheat, and a second one to pay for the fuel for burning the same wheat? Oh, that happened in Brazil with coffee, but then , as Charles de Gaule said, Brazil is not a serious country.