Yes, you should be able to post user notifications from a DeviceActivity monitor extension. If you are unable to do so, please file a bug report using Feedback Assistant. No, a NotificationCenter can only deliver notifications within a single program; it does not allow you to post a notification to other processes or receive notifications from other processes (i.e. you cannot use it to post notifications between your monitor extension and your main app).
Topic:
App & System Services
SubTopic:
Notifications
Tags: