I’m new to the product (but love it!). Am I overlooking something simple?
Goal: Delete a single connection from and item that has many connections. The only way I see to accomplish this is “disconnect” – which gets rid of all connections. Then I have to recreate all the other connections that were there previously.
Am I overlooking something?
Drag the node that you want to disconnect onto the one it’s connected to.
Note: I’m on Windows, but I presume it’s the same behaviour on the Mac.
It is. 
Mr X
You can also use the Disconnect menu command but first select both the notes you want to disconnect. If you select only one, all its connections will be removed, but if you select both only the line between them will be cleared.