How does the system know its the last in the chain ?

By no plug inserted in the 'continue' connector.

If a 'screw in' phone jack with 2 signal and 1 ground was available I would just connect the termination through the contact that closes when the jack is pulled.

The problem is that all connectors with switches I have found so far (thanks to responders to a previous post) are a bit too weak for my application.

Erik