1) Alert Central will stop the notification workflow once an alert has been acknowledged, assigned, or closed by a user or the group escalation policy. It is possible for a user to acknowledge and close an alert, even if the problem has not actually been resolved. Alert Central does not track changes to external systems.
2) Alert Central is different than Orion. Orion alerts will be triggered, and then go inactive when the problem goes away. Alert Central alerts go through a automatic notification workflow until there is user intervention or the group escalation policy finishes. Once the automatic workflow finishes, the alert will remain in whatever state it is in until there is user intervention.
3) The ability to configure more of the email notification templates including the subject line would be a great feature request!