"cannot iDivert – missing capability"
Mostly likely this is due to configuration on CUCM - I suspect there are several possible configurations or mis-configurations that could prevent the capability from being exposed to the Jabber client.
A good test would probably be to use the CSF device you are using for the Jabber SDK to register a Jabber for Windows/Mac instance. If you can use iDivert in Jabber it should work in the SDK. If that's not working, then you can review the feature configurations and checklist here: Cisco Unified Communications Manager Features and Services Guide, Release 9.1(1) - Immediate Divert [Cisco Unified Commu…
Your regular Cisco support channel (TAC or partner support) is the right place to go for CUCM config questions.
Once you get the Jabber client working, if you are still having issues with the SDK, I would recommend opening a ticket with DevNet Developer Support so they can troubleshoot with you directly, take a look at the detailed trace logs, escalate to Jabber SDK engineering, etc.: https://developer.cisco.com/site/devnet/support/