I didn’t think this would be possible, but I searched “white wash slate” on Pinterest and there were a few posts 🤔. Would be interested to see how it turns out. Good luck. https://www.pinterest.com.au/pin/251779435401045832/

If the designer is a good designer then follow their lead. They do things for a reason that others may not know about such as structural issues with large glass area's. Could have a large beam overhead. Also things can vary from the plan drawn to actual construction so they maybe erring on the side of caution so if there is a variation during construction it doesn't end up an expensive rework, just a slight site change only. Are the window and door heights 2400mm?

What country and state are you in. In NSW, Australia there are statutory warranties. https://www.fairtrading.nsw.gov.au/__data/assets/pdf_file/0019/382123/Consumer_Building_Guide.pdf

Try this website. https://onlinelighting.com.au/interior/interior-wall-lights/indoor-step-light/