Your comment about "all users have the same PortletPreferences" doesn't seem possible. Each user has their own PortletPreferences as per the Portlet Specification. I'm pretty sure GateIn can do all of the things you want on your list. Perhaps you can clarify what your problem is.
It seem possible in a User Page (Owner Type : user) but not in a Portal or Groupe Page. The issue is that User Page can't be managed by another User (manager:/corp/administrators). Each user has his own page.