It is true that CalDAV is default for Leopard users.
In my tests, I can make changes to any events in my calendars except ones that were created by others inviting me to meetings. I can watch things get shifted around in Zimbra <-> iCal (though it helps to use the iCal 'Refresh' command) the only other limitation that I can see is that it really helps to have iCal open all the time, even if it is just lurking in the background. I am not entirely sure why you are having calendar editing problems as it simply works on our production system, so I am limited in my ability to help.
To answer your last issue, you can't really sync local calendars from iCal to Zimbra via CalDAV. HOWEVER, I have had great results using iCal's File > Export to create a .ics file of the local calendar, and then importing it into Zimbra via Preferences > Calendar > Import, and then sharing it back out through the Sync connector.
Chris @ DPM
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