Can birthparents contest a finalized and proper adoption?

I think you may be asking two different questions. Can a birthparent file a claim to rescind their revocation of rights? No, not if state law forbids that. But can a birthparent contest an adoption? Probably, but they'd have to prove they had grounds to do so. For instance, they could claim they never signed any revocation of rights, that their signatures were forged. In that case, they'd probably be given a court date to prove it. And remember, filing a claim does not equal winning it. If you really do have a finalized and proper adoption, they won't win. It's only if you don't have a finalized and proper adoption that they might win. And that's what the courts would be charged with figuring out.