Probably not the right section. But to answer your questions - the only use of ARC is to get audio that originates at the TV (not the AVR) back to the AVR. Some implementations (I say some because I don't believe this function is guaranteed to be there) would also route audio received from HDMI sources down the ARC cable.
If you want to use the TV speakers some of the time, the path you suggest would be worth a try. The issues could be 1) the TV may not route audio from HDMI inputs back out the ARC output and 2) the audio received by the TV may not be the best quality possible - if the TV doesn't support DD5.1 decoding, you'd only get 2-channel PCM sent from the set-top box, whereas if it was connected to the AVR you would.
The solution to either of those while allowing the use of the TV speakers is to run a digital audio cable from the set-top to the AVR separately and use that path. There's no downside as your set-top isn't (currently) sending any audio "more advanced" than DD5.1, so the digital audio path will contain the best audio available. That would not be true of BD players, for example.
But try the ARC setup and see what you get. Use the digital audio cable as your backup plan.