I'll address these DFM group questions in this thread, but you will need to start another thread for the syslog question.
If you have a lot of networks, and you want to create notifications groups for each one, you can create new DFM user-defined groups instead of using the pre-existing Customizable Groups. You can also create user-defined groups under Common Services which can be used for DFM notification groups.
No, you cannot change the names of the pre-existing Customizable Groups in DFM. You can, however, change their descriptions. New user-defined groups can have any name you like.
I'm not sure what you mean by hidden group.
The Customizable Port and Interface groups are primarily used to configure different threshold policies. In addition to being used for notification groups, the Customizable Groups can also be used to customize polling policies. New DFM user-defined groups can NOT be used for threshold or polling policies.
Grouping all the trunk ports in Germany is not something that can be easily done since the Customizable Trunk Port group doesn't offer sysLocation as a matching criteria. But I'm not sure you really want to do this. The way these interface and ports groups work is to allow you to group certain devices, ports, and/or interfaces together that will have the same threshold parameters. Should all the trunk ports on all the Germany devices have the same threshold values?
DFM considers trunk ports to be any switch port which is connected to another managed device (i.e. an uplink port). Nothing needs to be done on the devices to make this happen. You just need to add all of the devices you want to manage into DFM. Any switch port that is not connected to another managed device is considered to be an access port, and it is not managed by default.