but what happens to PIN 9?
Nothing leave it disconnected.
This pin is designed to be used with motors that generate a back EMF, by connecting this to the +ve supply will put a reverse diode across the load. You do not have to connect it to anything.
I think I understand it now, just a quick check to make sure I'll be doing it right:
Now when I look at this scheme, I find it a bit useless indeed to use there transistors to controll one single channel. What is your advice, replacing them with 3 BD679?