Regarding question #1, why do you need the ARC?
The CATV program audio is on the HDMI cable as it leaves the Cablevision STB (set top box). The A/V receiver (Onkyo in your case) decodes this STB-generated digital audio stream sending sound to your home theater system. The HDMI video stream (and audio) is switched by your A/V receiver to the HDMI output connector associated with your TV.
The TV set not normally in the CATV home theater audio path. The HDMI ARC feature might be used if you are decoding audio from an ATSC (off-the-air digital broadcast) at the TV and want to back feed it to you home theater. In this case the STB plays no role.
Regarding question #2, the most common HDMI connectors for non-portable devices (like your setup) are the full size (Type "A"), as illustrated here: »en.wikipedia.org/wiki/HD ··· nnectors
I normally buy my HDMI cables on Amazon or from Monoprice.com, where you can normally find cables that are much cheaper (and better quality) than those at local retailers. 26 AWG and 24 AWG HDMI cables are better (lower loss) than the less expense cables made from 28 AWG wires.