Hi Mike,
I can’t speak to the particulars of Wordpress’ API, but I will say that if you’re working this issue through in your app and the simulator, a faster way to get there will be to use the awesome (and free) Postman app. You can make modifications to your API calls on the fly and see the response instantly. And once you’ve worked out the secret sauce that gets it right, just duplicate that in your Corona app via network.request.
You can get Postman here: https://www.getpostman.com
I’m not associated with Postman, just a fan who discovered it way too late. If I’d had it handy when developing the Twitter plugin, it would have saved me loads of time. 
At the end of the day, Corona as a client is no different than any other client. If you can successfully make an API call in Postman (or any other client), you can do it in Corona. Good luck!