We are currently being the witness of that situation on KosherMania's case, aren't we? They doesn't accept any of those documents, and the player and the casino are currently in a constant tug-of-war of verification.

Whether they'll confiscate the player's money or return it, I can not say, as we are yet to see the outcome of it. But from past experience, I will dare to say that a casino will not confiscate player's fund without strong supportive evidence. All they can do is to make the process difficult [perhaps borderline infuriating], but they will eventually have to return it if there were no basis of questionable source of wealth.

Unless, of course, the casino [or broadly speaking, any platform out there] itself is questionable.