My parents' old house had a wonderful oak front staircase that my six siblings and I used to race up and down, taking them two or three at a time, with only a casual hold on the railing. I don't know how we managed to never slip or fall. Now, the timber backstairs were steep, and had narrow treads and were poorly lit. We NEVER ran up or down them, but held tightly to the railing and hoped to heck we managed to stay upright. As an adult, my previous home had carpeted stairs, which I slipped on (wearing socks) and fell down twice, though thankfully I was closer to the bottom than the top. My current home has timber stairs with three narrow stainless steel runners stood on edge and inset horizontally across the front of each tread (this is supposed to help reduce risk of slips). I doubt I will get over the anxiety of taking another tumble, so I never go down them wearing socks, and if barefoot, I hold onto the railing for dear life and take it nice and slow. So far so good.

If you paint with a non-yellowing wood finish and/or perhaps a light coat of stain in your preferred colour you can then keep the same throughout which will make the house flow

If there is a storage cupboard underneath the stairs, then I think its best to let go of the open risers. At the end of the day when it comes to stairs, I think you need to make simple decisions, don't complicate it. Sometimes spaces fuss over stairs and they can look great, but they are very impractical, expensive and dangerous. I think if you live in a house with no kids and no elderly, then you can lash out and go for something with more design risk. I think the timber stair profile shown in your picture looks great, and would work well with your vertical timber beam balustrade. However timber stairs can be noisy and quite slippery. If you are using carpet upstairs then you could consider running carpet all the way down the stairs. Don't do the half half thing. In order to finish the edge of the stairs neatly, I would edge the stairs with the plaster finish continuing from the cupboard wall, all the way up to the edge of the stairs and making an edge profile for the carpet to be neatly tucked into. By doing this you could still get the stepping silhouette which would look very cool. Then the beams could run off the plaster surface and not the edge of the carpet. Ultimately carpet may be annoying to have to vacuum, but it is a lot quieter, considerably cheaper and safer than timber stairs. Hope that helps. Good luck with it.

all heaters and stoves should have very specific installation instructions including distance above the stove to any mantlepiece and i suggest you start with advice from local suppliers and then confirm with specific written instructions from each manufacturer that you can usually find on line...There are a number of heaters that have wood fires that are designed to be inbuilt in a purpose made wooden frame so hopefully this can be done inside your opening