[eluser]Randy Casburn[/eluser]
Here is what you should do...
Step 1) Make a Clean install of CI and get the Welcome page to work with your web server using a basic HTTP_AUTH capability and nothing else (NO XML-RPC, nothing else)
Step 2) Refer to the excellent CI documentation on how to set up a CI XML-RPC server and Client and do that next using the tests in the documentation and get that to work perfectly while still using your HTTP_AUTH from above
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Once you have accomplished those two things, you essentially have accomplished what you want to achieve. Once you get this all working, its a matter of accepting the incoming requests from your clients and using CI controllers to direct those XML-RPC server requests through the CI XML-RPC server to your PLESK API.
The CI XML-RPC server should be able to do this.
Randy