I am evaluating Zimbra Collaboration Suite with different clients (outlook and thunderbird). I am evaluating following three features

1) Ability to subscribe to an external calendar in iCalendar (.ics) format.
2) Ability to publish/export a calendar in iCalendar (.ics) format.
3) When enabled, changes made locally to Address Book, iCal data or to Entourage calendar and contacts are immediately sent to the Zimbra server.

I am little bit confused if the calendar used/shared by a user using Firefox will be properly understood by the above client.

Here I think the features related to iCalendar are only supported by the Zimbra Web Client and the Zimbra server stores the calendar information in proprietary format.
Is this the case or does Zimbra server properly handles/understands the iCalendar requests from above mail clients?

Will an outlook user able to share an iCalendar and a Zimbra mail client understand it?

Is there a feature in Zimbra server to disable iCalendar support? This may help Zimbra web client users to dis-allow other mail clients users to subscribe to iCalendar share's.