Useless pins in HDMI cable?
Which pins are not connected? All pins should be connected. But as the TMDS clock and data pins are differential pairs, they run as twisted pair where the shield is connected to ground. It might be that the diff' pair shield is not actually connected to the respective ground pins but to the connector outer shell. I would not count on having 19 separate connections between connectors. Basically, as some pins are differential pairs, some are shields for differential pairs and some are just standard wires, it is a bit tricky to determine which pin should carry which signal.

But just to avoid confusion, I do not recommend using connectors of standard interfaces for any custom purposes like this. What if someone plugs a computer or monitor to your device and damages something.