... And if someone used my public domain software in these jurisdictions, who would they face legal threat from?
One threat is from heirs of the original author, who claim that because the coypright disclaimer is unlawful, they retain the copyright and are therefore "owed" a lot of money.
Is this related to estoppel? Even if the copyright disclaimer is unlawful, it was clearly intended by the copyright holder, and I don't see how a person could seek to benefit by showing that they are in contravention of the law.