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You wouldn't need an app or other WhatsApp users beyond your distributed proxy accounts. You'd be running the monitoring through these proxies.

Creating an app with the sole purpose of backdooring WhatsApp on a user's phone seems like it'd open you up to a lot of lawsuits. Ethically its a mite more questionable, but the original article is still unethical in that you're monitoring people without consent.

Like I said above, I'd do this just so that they'd crack down on it. It's still a "means justify the ends" argument, however, so you have to be quite comfortable with moral relativism.