Beginning migration to 2013

You can migrate user's mailboxes from Ex2k7 to 2k13 and place 'SynchInMigrtion' mode dword value 1 as directed by Adviser, you can take reference information from http://www.symantec.com/docs/TECH48928

EV servers needs to be published for EV action if users accessing via OWA(OR RPC anywhere). There are configuration changes as EV work with OWA 2013 only via office mail app. So you need to configure mailbox and define external-internal EV URL as well.

The details on configuration steps can be found in TN http://www.symantec.com/docs/HOWTO83364, it has few excellent video to understand how app work with Exchange 2013.

EV icons would not be visible in case of Exchange 2013 OWA access so icons should not be an issue but Barracuda should not stop other functionalities. You may wish to talk their support representative if anything breaks.

We can utilize single internet publish record if Barracuda allows web proxying. (we don't have any sort of certification for Barracuda as firewall). The rules & functionalities should work as mentioned in link below (this if UAG firewall).

http://www.symantec.com/connect/blogs/publishing-enterprise-vault-10-microsoft-forefront-uag-2010