Yes, ARC enabled HDMI should carry audio back "upstream" from the TV to the AVR for processing. A separate cable is not required. You need to go into the Panny's menu to it's HDMI control section (Panasonic call it Viera Link I think) and enable it's audio function. You may have to do similar in the Denon, so it recognises the Panasonic as an (audio) input when required, as well as an output. You may have to switch everything OFF then ON again to get the HDMI to "handshake" after you make any changes to HDMI control.
If you enable full HDMI control on the Panny and Denon, you should be able to hit the ON button on any input device (including the TV) and the device, AVR and the TV will fire up and all the correct outputs and inputs will be selected automatically.