It seems that you are having some issues with your Slingbox PRO HD since it does not stream out the video signal.
Even though you have applied several tests and troubleshooting steps, I recommend you to get rid of the passthrough and use a Slingbox- cable box direct connection. This tests will allow you to determine if the issue is related to any Slingbox component. If the issue persists, I recommend you to use the S-Video cable and make a final test.
As a final option, I recommend you to contact our technical support department. We will provide you with the available support options, according to the Slingbox activation date.