Like stated above, you need to be sure this is just one layer of decorative brick with just drywall behind it. If it is one of those fireplaces that the chimney is brick and all, you have a bigger issue. If just decorative, you will need something like we used to remove our faux rock. We got a little pneumatic attachment for our aircompressor. It lookes like a large screwdriver or mini jackhammer. Starting at top, you chizzle away. This is very time consuming but can easily be done. There will be lots of dust and debris. When all you have left is the studs, you simply put in new drywall. Again, this is all easy, but since this is a fireplace I am not sure what safety issues are involved like if you need specific materials on wall because it is a fireplace. My regular firebox just had drywall and so does my zero clearance. Also as stated above, there may be issues with what kind of mantle material you can safely use and at what distance from firebox. 12" is pretty standard but depends on mantle material. We lowered our mantle but we also brought firebox down to floor level. Rebuilding it was not that difficult. We simply built the legs, built the mantle and hooked them all up. The cost of redoing the brick is up to you. If you do the work it is just materials. But obviously hiring someone to do this will cost you. I would imagine mortaring brick is very time consuming because they are so little. But a seasoned professional can go pretty fast. But again, it will cost you. Sounds like what you want to do is doable and can vary in price depending on who does the work.

Mortar has its lowest strength in tension. Hold the chisel almost flat against the brick wall and directed at the mortar joint of the brick you want to remove. Once the first on comes off the rest should be a lot easier. You may need to use a plugging chisel to remove the mortar on one side of the first brick so it ha a way to move and break the bond.

I would try a heavy bodied ( paste) chemical paint remover to soften the paint, and then remove the residue with a vigorous spray with a power washer. You might want to then wash the entire house using a cleaner intended for decks which contain oxalic acid. The Behr Deck Wash from Home Depot does a good job at cleaning masonry and concrete.

I suspect that a lot of that is sheetrock on a 2X4 framework. It wouldn’t make any sense for that to be a massive brick work underneath all the tiles. if you wrap on the tiles with your knuckles you’ll probably hear a more hollow sound rather than something solid. The asymmetry of the wall with that large area to the left suggests that a lot of the wall is a facade rather than structural. Not knowing how the mantel is mounted, I can’t explain to you the best way to remove it, but that would definitely show you what’s under the tile. The mantel could simply be supported by a couple of steel rods coming out from the wall, and even be pulled directly off with some effort; I just can’t say. Can you see any screw holes or wooden plugs in the two pieces that support it? As for saving the tiles, it would be much easier to get them off the wall intact if they are affixed to sheet rock. Regardless, it would involve a bit of work to clean off the back sides so they could be reused and made to sit flush with other tiles elsewhere.