EV extensions via OWA failing but only via converged FQDN

FYI for anyone happening on this post - problem solved, eventually quite easily, but it's difficult to tell if it was the sole problem due to the number of other changes. Here are the bullet points, so hopefully it will provide some inspiration for others troubleshooting similar problems

Problem:

The EV addin\link is not appearing in OWA

EV version : 11.0.1.3706

Troubleshooting and eventual solution:

The Symantec Enterprise Vault OWA 2010 Extensions Version : 11.0.7570 is installed on all Client Access Servers

When we hit the cas's via individual server FQDN's they work - cas1X.domain etc

When connection is attempted via converged FQDN - OWA.domain then no extensions are delivered

Tested from outside of LAN, no Enterprise Vault shortcut present

Basically I ruled out everything bar proxy config as the root of the problem

Next I aimed to clarify the steps appropriate config as per the 'Setting up Exchange Server Archiving' guide (see page 183 of the linked PDF below):

https://origin-download.veritas.com/library/BUSINESS/DOC6282/10.0.4-Setting_up_Exchange_Server_Archiving.pdf

Checked the exchange servers.txt file, it contains all IPS of the CAS', but the Converged IP was not there nor needed as this config used Round Robin

ISA rule looks good

Authentication is NTLM

Ensured the Configuration for Exchange Web Services impersonation on CAS proxy servers was correct

Enabled the impersonation management role and added the OWA site to the compatibility mode and all started working