There's a gotcha in that port 2304 needs to be opened on both firewalls, plus your ssh port(s).
After updating authorized_keys..
Appears to work fine.
When you create/edit the API key, in addition to clicking CWP to CWP, ensure that you also click the appropriate entries in the List column i.e.
Account
Account Details
Account Quota
AutoSSL
Cronjobs Users
Domains and Subdomains
Emails
Ftp Manager
Metadata Account
MySQL Databases
MySQL Users
Packages
Quota limit