Tried latest Apple Calendar Server solution and found that they use a proxy mail account for invites and responses.
This works quite well and avoids the problem that i outlined in ticket 0004425.
In Apples Calender Server one configures a global mail account for the server as well as mail-addresses for the users.
when now some user sets up a calDAV account in iCal with the Apple Calender Server and creates a first appointment with members,
the invite is sent via the global mail account and responses to this appointment are returned to this global account.
So all parsing is done serverside and any status of members is directly rendered within the appointment.

I tried to call Apple for the problem that parsing ICS-attachments via mail does not work as intended but surely they said it's a
third party problem...

It would be convinient to have such a feature in SOGo as well