Hi Pieter,
Your second configuration is the correct one. To avoid the need to select a path, you could set isPath="optional" (altough that migth give issues with entityProperty, not sure) or introduce an additional, not required property.
Your widget could check at runtime that if an path is set, a child object should be set as well, and vice versa. (The widget XML is not very flexible, so you cannot express all logical check that you need.)
I still do not understand you completely. You show a piece of XML that defines the interface to the modeller. However in the modeller only metadata is selected unless you use xpath constraints.
If you want to be able to select tasks and activities in a widget you can do xpath retrieves via the widget so it is possible to choose a task and get all activities for that.
Maybe you are locked in the way you want to have it, but is there another way around. Can you tell me more about the goals you want to achieve?
Maby someone @ Mendix could give a go at this? :]