Finishing around brick veneer fireplace

The keys are the "key". When real plaster is applied over wood lath, the base coat oozes through the gaps in the lath and sags or expands a bit forming "keys" that, when dry, hold the plaster layer to the lath.

This is a strong structure unless a substantial amount of the keys have been broken (from temperature or humidity swings, structural shifting, physical damage). When this happens in a specific area, that section of plaster is basically unsupported and is prone to cracking and buckling inward.

If the plaster is generally sound, you should have no problem. While drywall is more flexible and less prone to pulling away from the framing, sound plaster should be strong enough to hold the weight of a thin veneer.

If, however, significant sections of the plaster are bulging outward, or if there is a large amount of the surface coat pulling away from the base coat (a sign that the plaster is in poor condition overall), this is not a good basis for a top coat beyond paint or paper. In that case, you may need a more significant redo.

If the weak areas are just a few and not too large, there are specialized plaster washers that are used to screw the weakened section to the lath to prevent further damage. That may be sufficient if the areas are few and far between.

1) you could use wiremold. They make 90º inside corners. I don't like it but if you have use it then it works. Better off to chop a new box in the wall exactly where you want it.

2) There are several ways to put power wiring behind the planks. EMT, ENT, wiremold...there are others less often used. You will need an extension box on the left power box to run out of.

3) The bricks around should just be a facade but why make it hard on yourself? Just bring the TV mount area out far enough to have the media box behind it.

4) Putting power in the back of the firebox will probably be a major pain. I believe that is firebrick surrounded by other brick. You may need to get a bricklayer in on this. Or at least you should study the construction of fireplaces.

I see other problems here, though. The National Electrical Code requires that no point along the floor line be more than 6' from a receptacle. If you cover the receptacles on left and right will there be other receptacles on the wall line close enough to satisfy that requirement? If not you will have to add them. Also, you can't cover junction boxes they have to be accessible inside the cabinets.