What To Do

What about some 3/4" plywood behind the veneer. Then you can put bolts wherever your like for your mount. You'll still be faced with drilling into your stonework though. How high is he ceiling? Any chance of a ceiling mount of some kind?

Also, you're TV is going to be pretty high to begin with, if your ceilings aren't too high, maybe an in-ceiling center as well? It's certainly not the best setup, but you're forced to make a compromise between having the TV higher and having that center channel where it should go.

Another thought, what type fireplace is that? Any idea on what the clearances are on combustibles? If the mantle is high enough, you might be able to mount your center under it. It's going to get warm, and it will likely cause premature failure, but if you're not looking at high end speakers, it might be a tradeoff you're willing to make. That's sort of a long shot, but it might be worth looking into.