Workaround: The 5.0.2 mobile web (
https://zimbra.example.com/m/) will show all mounted calendars, though it currently can't modify them.
There's not a lot that Zimbra can do about a calendar application on your phone that only supports one calendar, and an ActiveSync protocol defined by Microsoft that only supports one calendar. The suggestion in 7256 is to merge them and present them to ActiveSync as a unitary calendar. That could be fine for read-only access, but it's not at all obvious how to handle phone-initiated changs to calendars other than your own.. especially since there will be no way to tell the phone that certain items are read-only.
Helix is 6.0, not 5.5. By that time, there *might* be a new version of ActiveSync that handles multiple calendars.