To keep from updating when the screen is off, use the AlarmManager to schedule a recurring alarm that doesn’t wakeup the phone.
The other two bullet points you have in your question aren’t possible. There is no way to detect if your widget is on a home screen that isn’t currently visible, and there is no way to determine if an app is running that is hiding the home screen. I have filed a ticket on http://b.android.com requesting this functionality be added to Android. If you feel like starring it, it will help it gain priority: http://code.google.com/p/android/issues/detail?id=5529&q=reporter:mark.r.baird&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars