I would also check the OS of all the BB's involved.
What is the workflow for this group of users, do they all manage their own calendar or do they have a delegate(s).
If they have a delegate - critical to check "send all meeting requests to my delegate not me" in Outlook. If not they can delete the meeting and over ride anything the delegate has booked / updated.
Also you didn't note but have you applied the MR for BES? Each has had wireless calendar tweaks.