My opinion would be to stick with the character of the rest of your house. If you have Coastal and Craftsman features, don't go too Contemporary or Formal. Both examples that you have posted. ;) Besides the siding on the outside of your home, do you have any hard surfaces, like brick or stone? If so, match those instead of bringing in a third element. We recently built an outdoor area for our home. I had my heart set on a stone fireplace, but the house is brick and stucco with a wooden screened porch. The more that I thought about it, the more I decided that bringing in another element was too much. Matching brick would be the way to go. I am so happy with my choice. We also have columns that mirror the columns on the inside of the house and painted to match the screened porch trim and stucco accents. A cedar wood pergola is stained to match the darker taupe of the majority of the stucco. All colors blend in with the brick. If your house is all siding, then I would go with the stone. for inside and out. I believe that the plaster both outside and in, as shown in photos 2 and 3, would be too contemporary and formal for your home. I like the interior stone and wood trim photo that you posted with the hearth, but sadly, you do not have room for that set up. Another opinion...I'd take down the knee walls. You don't need them for a division. The one between the window is taking up valuable wall space. You can create a natural division between the dining and living space with your furniture. I know from experience. We put a built-in in our living room, and I wanted it to have some character, so the middle section sticks out about 10 inches beyond to side sections. That middle section has become a thorn in my side and prevented a couple of different seating configurations that I would have much preferred in that room. The bump-out gets in the way. I think that your knee walls will narrow your walking space, getting around furniture, such as a sofa. Again, just my opinion.

MY suggestion is you make a trip to the place supplying your FP and no to this type of purchase on line. Here we do not even know what the FP looks like for me a see through is not a choice I would make but at least post a pic of thta . Cast stone is a funny name since you do not cast stone but we do not even know your style .

Porcelanosa has thin porcelain slabs in sizes you need. Used on my fireplaces. The problem might be finding good fabricator https://www.porcelanosa-usa.com/products/tile.html/porcelain-panels/liemdark-nature

If your fireplace is the same as mine, the only way to "update" the green marble is to peel off the mantel and install a different material, either marble or granite of a different color (remnants at a slabyard), or tile. Mine is from 1995. The marble was put on first and then the mantel on top of it, so you can't remove the marble without removing the mantel. My mantel is the same as yours, except white, so I suspect yours is the same vintage. After removing everything and choosing a new surround, you would apply a new mantel as removing the existing one would probably damage it such that it can't be reinstalled. I also have green marble, although mine is so dark as to almost appear black. I am planning on changing it whenever DH gets around to it. If you can DIY it's not too expensive although still several thousand dollars. Once you remove the old materials you can do any number of things. Start browsing the pictures on Houzz to figure out how you want it to look. There is a category in the Houzz pictures for fireplaces. Do you want a less traditional look, or a much more modern look?