Hi,
Each OpenGear card will take up two slots due to the rear panel of each. So indeed a 20 slot frame will accommodate up to 10 Blackmagic cards.
The OpenGear cards are like the Mini Converters and do not have reference inputs, and do not take reference from the frame. The one exception being the OpenGear UpDownCross which has it's own reference input. Of course the Sync Generator has 10 reference outputs.
Embedded audio will pass through the UpDownCross, SDI Distribution and SDI to Audio. The audio is not stripped.
The audio connector used is a WECO 3 pin connector. These are included with the OpenGear card, but you can find the one
here if you need to.