I’m really sorry to hear that this was not resolved. Instead of moving to NGINX, you can also try to deploy another WordPress with Apache and start the configuration from scratch. Once the domains are configured, you will only need to migrate the data using this guide
If you really want to move to NGINX, I suggest you use our WordPress with NGINX solution.
The links you posted are the correct ones but you can also take a look at this other one
I also suggest you use this other link instead of the one you posted
This way, you will have one WordPress running in the instance. You can configure the nginx-vhosts.conf file of that application and ensure it continues working as expected. After that, you will need to follow the guides to install a second WordPress and you will need to configure the nginx-vhosts.conf file again to set the domain of that second installation.