Okay, here is what I would do.
Resale. Before I start any project that is the first thing I think of. Will what I’ve done be practical for a potential buyer and, if not, what do I have to do to restore it to its original appearance.
I see no reason to build up or use a mount. Fireplace switch and gas valve solved. I believe someone suggested building a stand and sliding it in the alcove. You’d have the TV on top and your equipment on shelves below. That is about as simple as you can get and I would agree with that; only I would do something a little different. I would extend the stand a foot beyond the fireplace and run it to the end of the wall where you have your table. I think a 4X8* sheet of ¾” Luan would be long enough. Remove the table and place your sub (I believe that is what I see by the table) in the corner. The top of that shelf extension would be flush with the windowsill. The shelf should be wide enough to place your lamp and whatever else you’d like on it. On the shelf below you could store CDs, DVDs, place your front R speaker and sub, or whatever you like there. That would make that area less cluttered and more open. The rear of the stand that is tucked into the alcove would need to be secured because it would be front-heavy. By extending the front of the stand, this would allow you to shift the TV toward the window and away from the fireplace giving you a little more balance on that wall. The only thing I don’t like about this idea, and you and your wife may agree, is that with it being extended by a foot it would not be flush with the fireplace. A little design creativity on that end could soften that look so that it wouldn’t be so squared.
When you’re ready to move; remove the screws holding the rear in place, slide it out, patch the holes, and you are done. Just my 2 cents.
*This would be one solid piece that would be notched to slide into the alcove. Materials to do this entire project would be somewhere between 50 and 100 dollars.