It seems that the CalZapDav works well on rearranging activities (in time), and I would like to rearrange not only the meeting and tasks but the project emails, phone calls, etc too in time. It is possible to use the CalZapDav gui as a timetable to "rearrange" activities in time? What is the best practice to do this kind of work?
Yes, it is possible. The openCRX CalDAV adapter allows to manage all activities with icalType is VEVENT or VTODO (you can verify this with the standard GUI in the 'Details' tab of an activity). Therefore, CalDavZAP can be used to re-arrange these activites in time.