Hey Gary,
I understand where you’re coming from, but Discourse is not intended to be used purely for authentication purposes. I would suggest you spend some time considering a dedicated auth service which you connect to your various Discourse and Wordpress instances. A bit of investment in research now can pay off down the ine.
It’s relatively straightfoward. For example if I wanted to make try.thepavilion.io as the provider for test.thepavilion.io.
Step 1. Set up try as an SSO provider
Step 2. Set up test as an SSO client
My primary piece of advice to you though is that what you’re proposing is complicated, and regardless of the path you choose you need to set up a staging environment that mirrors your intended production environment to experiment with different configurations before you roll it out. This kind of setup can be affected by a number of different variables and it’s hard to give advice about it in the abstract.
I know the thought of setting up an entirely seperate staging environment for an interconnected web of servers may seem laborious, but the labor involved in that is alot less than trying to work through unforseen issues that arise once you’ve launched something like this.