You can post a new topic, or reply to someone else's posting. By the way, public folders aren't really public. They're accessible only to your workgroup. You set the permissions easily in Outlook. You can grant or deny access to any particular folder, to any particular member of your workgroup.
Security is assured.
Here is a screen shot from Outlook showing the permissions dialog for a public folder. Yes, you can use public folders offline, or online. If you set a particular public folder for offline use, Exchange will synchronize it when you check your mail. Everyone on your team will automatically get the latest information. This means your delegate can review and edit your calendar at any time.