We haven't actually bought Zimbra either, so this is a pre-sales question (hint).
Is it even possible to do #2, administratively shove a bunch of events into a user's calendar? The REST interfaces all seem to assume that you know the user's password.
We are able to do
https://webmail.carleton.edu/calendar.PNG today and would hate to lose that functionality. Admin staff who use calendaring every day might be motivated to manually enter global events that affect them. Students and faculty won't, and will need to be provisioned. While they don't know it, this has an effect on the squeaky-wheel admin staff as well -- imagine your a staff person trying to schedule your student employees, or a dean scheduling a faculty member for a committee meeting. That faculty member doesn't normally use online calendaring and has no real motivation to keep it up to date; but it would be pretty dumb if the system didn't know about his course schedules. We have hear from many faculty that online calendaring is not a productivity gain, but a shift of work from the secretary who used to schedule their meetings to the individual.