Is there any possibility of putting the TV on a different wall?
I suspect that reflections off that jutting wall combined with the absorption provided by the opening to the dining room are likely to make the sound somewhat strange. What is the room like toward the right of the fireplace? I suspect its shape is quite different in that direction, which is likely to make the soundfield seem rather lopsided.
So far as the speakers themselves are concerned, 3-4" woofers are going to going to require a high cross-over frequency, probably 150 Hz or even higher. In other words, frequencies below 150Hz will have to be handled by the subwoofer. With frequencies that high, most people can easily tell what direction the sound is coming from, so some will seem to be coming from wherever you put the subwoofer and not from the direction of the TV. Putting the sub in front of the fireplace is likely to be a point of contention for those who want to enjoy watching a fire.
I don't see any mention of stands. Where and/or how were you planning to mount the DefTech surround speakers?
To be honest, I suspect that speaker system might be OK for listening to background music, but not so good if you expect to do any serious listening for an extended period of time.
Edited to add:
Cat-6 wire is primarily for networking. You'll need it to connect the receiver and TV to the outside world, both for firmware updates and for listening and watching streamed entertainment services. Too often WiFi connections are just too intermittent, causing audio and video dropouts. Another use for Cat-6 is for HDMI if you get special adapters. It's often used when an HDMI signal has to be sent more than 50 feet or so, e.g. between rooms.
Marantz SR7009/7.1.4/FH+TM/DefTech PM1000/LCR+TM amped