australia
The legality of the arbitration and the non-disclosure clauses are independent of each other
As a general rule, arbitration is both legal and encouraged as a way of reducing the burden on the courts. They are rarely set aside and the avenues for challenging an arbitration decision are narrow. One exception in Australian law is that an arbitration clause does not prevent participation in a class action.
NDAs are also legal and enforceable. However, they cannot be used to protect illegal behavior or the reporting of a crime.
In the cases you describe, we have instances of illegal behavior and criminality.
In the first case, we have an ostensibly valid arbitration agreement and NDA. In the second, it is likely the entire contract is void an initio because it was entered for an illegal purpose (scam); that makes the analysis trivial- go straight to court and say what you want to whoever you want.
Going back to the sexual harassment case, it is likely that a properly drafted arbitration clause would be upheld and any claim for damages would need to be arbitrated rather than litigated. Arbitration is only confidential on the parties (as opposed to the arbitrator) if they so agree - in the arbitration agreement or otherwise. Note that any criminal prosecution is undertaken be the Crown in parallel to any civil restitution.
The NDA would not prevent reporting the matter to the relevant authorities for investigation. However, they would generally prevent disclosure to any other third-party.
However, NDAs in relation to sexual harassment/assault have recently come under both judicial and legislative scrutiny. The position is in flux but it is likely that a pre-existing NDA will not be enforced to prevent disclosure of such matters and post gov NDAs should only be established on a case-by-case basis and only be enforced if they are in the best interest of the victim and not for the sole benefit of the perpetrator.