You can do it the way you suggest although it may be difficult to attach the plywood (near the firebox opening you can use cement board which meets the code requirements for non-combusitble materials). However, I would suggest that you use mortar, and trowel it on in a sufficient quantity so that it fills up the irregularities and mortar joints and you end up with a reasonably smooth surface. You may have to do it in two coats - first partially filling in the recesses and then a finish coat - the reason is that otherwise a thick coat may sag before it dries. Then you will end up with a surface that is perfect for adhering stone or ceramic tiles to (and it won't stick out into the room as far as a plywood overlay), or even dry wall if you use lead or plastic anchors. This assumes that the existing stone has not been coated with something that will cause the mortar not to stick. If it has, you can use fortified thinset mortar, although it is messier to work with. Hope this helps. Also, since it's 15 feet high you will need a sturdy ladder or even scaffolding (cheap to rent) since the mortar is heavy and you need leverage to apply it.

I have two carved (solid) white marble fireplaces. One is completely unsealed and the other has a sealer that was done by the installer. I'm not sure what kind. The sealed one has gotten stained on the hearth, which is not a raised one. The stains look like pitch or tar-brownish. I suppose a marble restorer could do something to mitigate them, but they would reappear with future fires. The marks are concealed by the fire screen, which I keep in place to prevent flying sparks. They don't really bother me because we actually use the fireplaces for heat in the winter and I think they look better when actually used. However, I'm not sure whether the sealer makes the staining more or less likely. The unsealed marble fireplace has no stains. We also have a raw stone fireplace. It has more stains than either of the smooth marble ones. In my old house, I had three gas marble tile fireplaces. I converted the fireplaces to gas logs specifically because I was worried that the grout lines would look bad if ashes ever got into them. So, I never made real wood fires with a marble tile fireplace. If you are meticulous about building your fires far back into the fireplace, and about cleaning out ashes, you'll probably be okay. However, if guests or house cleaners or the otherwise uninitiated smear ashes, it would be difficult to get the grout to look like new.

That's a beauty. I would definitely not black polish the whole thing. It's hard to tell from the picture what all is going on there but the center part, as is, enhances the perspective aspect of the casting. I think I would do as little as possible to that center panel. To my eye the stark white has to go. And is that silver metallic around the outer edge? Is the main color gold or bronze? If that gold/bronze is chipped or needs renewing you may have to try several colors to get the right one. I would do nothing now. This is something to tackle when you have lots of time. I've done well by going to an art supply store that is staffed by knowledgeable art students (not a craft place like Michael's but a real art supply place where art students shop). I've used art brushes (on edge so the paint is only hitting the high points on the casting to make them pop) and various acrylics including gold metallic to get subtle antique-like effects. I recently refinished some Eastlake hinges and shutter latches using that method. My point is, don't do anything until you have time and then approach it cautiously. If you must clean it now do not use water. Try paint thinner in a small outer area and if there is no damage use that to wipe it clean.

Vicostone is typically less expensive because of cheaper labor in Vietnam. I have Vicostone under the name of Pentalquartz in my kitchen. An 8' long vanity with two sink cutouts will be an expensive top. It will be hard to find an 8' remnant but it wouldn't hurt to call around. Can you downsize the vanity to save money? You do need at least 60" for double sinks but a 48" single sink vanity is adequate for a guest bath. You'd save on sink, faucet, counter and cabinet. You'd have plenty of room on the sides for cleaning the floor even with a 60" vanity.