From what I've heard (and we don't use the fireplace is why I considered this a small risk - if we were to use it again, I'd probably install a set of vented gas logs, and add a fan to the firebox to try to push the heat out into the room...), is that you should try to light a fire, and put a thermometer above the mantle where the TV will be located just to prove to yourself the temps. If you're not going to do it before this winter, just see what it registers first?
Supe - I ran the cables in the chimney chase - picked up a few of the "access" flush mount panels that fit between studs and mounted one behind the TV and one in the side of the cabinet. When I stand on a stepstool, I can reach from one hole to the other to run cables. I added a hook inside the chase on the left side and basically hung the extra cable coiled up inside the chase.
I was glad that the installer left the chase so open from the attic, it was pretty easy to climb inside, run wire, and cables, and beef up the studs behind the TV for mounting.