It appears that you are trying to create your own control panel. This won't work with Plesk (or any other modern control panel).
If you are using Plesk then
ALL operations such as creation of domains/subdomains, databases, database users, mail accounts etc.
_MUST_ be made from Plesk, otherwise Plesk won't know about your modifications.
Think of Plesk like the "master brain" on your server: It holds the "truth" (aka. the master configuration) in its own database and will then configure the services on your server (config files etc.) using this information. Any modification in Plesk will trigger the configuration updates on the server.
This process won't work the other way around: Plesk will not pick up modifications of the configuration that you've made manually or that were created by another script.
However, the key feature of Plesk is exactly what you're trying to do: It allows each of your tenants to configure their own users/databases/subdomains etc.
You are the admin of the server and you create customers. Those customers then can manage their domains from Plesk without any actions from you. Isn't that exactly what you want? So why don't you use this key feature of Plesk? You can even create resellers that in turn will manage their own customers.
Have a look at this, it will explain how all of this works:
Customers and Resellers