I too am looking for the discovery logs- that's seems like the best place to mine created vs found vs updated from specific discoveries. That could easily be pushed into custom properties via rest.
Currently I search discovery profile config XML for matching IP and assume it's one-one.
There is currently no direct way to connect a new entry in Orion.Nodes with the corresponding entry in Orion.DiscoveredNodes. Matching by IP address and EngineID is about the best you can do. It's a good idea though.