Hi Bjoern,
To be able to synchronize meetings from external was one of the main goals for Desktop Connection. Regarding your 1. question: yes, synchronization of external (send by a customer) appointments together with "Desktop Connection" is possible.
2.: Updates regarding some of the CRM fields can be performed and synchronized, e.g. update Account in CRM. But you are right might be better to set Desktop Connection appointments read-only.
Kind regards,
Silvia