I understand you have gotten an answer to your question already from our developers, but for anyone else:
1) Yes, the NotificationEndedNew action occurs when an instance ended because a new instance was created.
2) Yes, the instance ID associated with the NotificationEndedNew action is the newly created instance.
3) As of the current version (1.1.1200.35) and the soon to be released version of Notifications, the instances IDs are sequential, so you can subtract 1 to get the recently closed instance ID in this case. Obviously, if there is no previous instance this will not work (e.g. the first NewNotification event action on a notification).
I believe we will update the documentation to make it clearer that NotificationEndedNew is associated with a new instance, not the ended instance.