Should I breach peer-review confidentiality if I suspect misconduct and the editor is not willing to pursue it?
That's a tricky question. I will try to give you my humble advice.

First, document every step and be sure that your statements are true. If you have some suspicion, you can also present that, but state that you only have such suspicion.

Second, write again to the editor telling him/her that in good faith you believe the study is fabricated, and it will undermine the journal credibility. If you do that after publication, the damage to the scholarly literature will be even larger.

Third, tell the editor your plan. For instance, you can tell him/her that if he does not act swiftly within 30 days, you will inform, without divulging too much details, of the situation, his/her insitution, as well as the authors' institutions and the publisher.

Fourth, if the above does not work (it should), then proceed and be ready to defend thoroughly your argument.