I would leave the brick alone. I think it works well with the color of the floors, and brings in some warmth to a cool palette. It also pulls that rusty color out of the artwork above the mantel. As for the surround, my inclination would be the bronze. I am not sure that I would like the nickel or chrome as well with the brick color. I wonder if you would consider putting the two candlesticks together one side of the fireplace, rather than splitting them. I think they are very nice, but would have more impact as a pair. Then I would get some birch twigs and put in the vase, to mimic the branches in the art. If that fits, you could put that on the other side of the artwork, with the two candlesticks together on the opposite side. Otherwise, it could sit on the hearth on the side opposite where the candlesticks are on the mantel. You have a lovely home.

We are also doing a slate fireplace surround, though we're having a contractor do it. I had the same question initially, since in most tile installations, there's usually a bullnose piece at the edge. However, our installer said that he'll put caulk on the edges that matches the grout color. From my understanding, you'd want a caulk to match the grout, since you want caulk where you have 2 different materials meeting. In our case, we're going with a sanded grout and a sanded caulk of the same color. Below is a link to a picture of the slate we're going with. Can't send a picture of the fireplace, since it won't be done for a few weeks. Here is a link that might be useful:

Typical porcelain tile in the 5/16" to 3/8" thick range will weigh in at roughly 3-1/2 to 4 lbs per sqft. Your 20x20s weighing 10 lbs? They fall right into that range. So your first question, no, the tiles are not too heavy for the walls. Or for the modified mortar used to adhere them to the walls. As to installation issues, yes, the flatter and more plumb the walls are, the easier the installation. Thinset coverage? The installer will want to set a tile and then pull it off the wall, check thinset coverage, and adjust the trowel size if needed. If the walls are flat he can use thinset. If the walls are wonky and a thicker setting bed is required to keep the face of the tile in plane, then the installer can move on to a medium-bed type of mortar. Setting mortars come in low-slump formulations to help prevent tile creep, but shims and tape can be used as well if needed. Tile waste? Figure out a pattern for your exact wall size considering the exact tile size and the exact grout lines you want. In a perfect world with 20" tile, three tiles on the back wall and 1-1/2 tiles on each side wall will cover a 60x30 tub surround nicely. Added labor with large format? It's all in the prep. If the walls and corners are plumb and in plane, then the installation may be a breeze. If the walls are wonky, then to get a great installation, the pace of installation can slow dramatically.

I can't enlarge those pics enough to see, but it appears so. you can't put wood next to the firebox, so i'm guessing they've polished the edges. all you need is some sandpaper from 220 up to 800 or so. I've done it. marble is very soft and you can put a honed finish on it pretty easily. if you want it shiny polished, you'll have to graduate up to higher grits. I'd stick w/a honed finish,,,no one will see it. You could also do chair rail molding for the outside if you didn't want to do wood. or the marble pencil liner that lays flat would be another option. I like your kitchen. are you doing the same tile as your backsplash? I'd do something w/some black for more contrast. maybe cement tiles, or the darker marble w/the nero marquino. like the chevron tile in my niche. that would good as your splash. this splash in a herringbone would look so cool..I'd take it all the way up to the ceiling in your kitchen, and above cabinets.