I look forward to making you successful with video, it’s not easy but very rewarding and fun!
Good job finding someone to do the soldering for you.
Good find on the ADC, no, I have not seen that. Should be fine and allow you to sample at higher frequencies if you need to.
The PropCVCapture is flexible with regards to pins as you’ve found. To stream data to ViewPort only the 4 MSB bits are used to reduce memory usage on the Prop. I’ve found that it’s best to do video step by step- starting with confirming that the ADC is properly reading slowly changing analog values, then measuring the NTSC signal, and then using PropCVCapture to view a video signal.
I published a guide here:
Your schematic and ideas all sound good, keep us posted on your progress/success!