Is this a prefabricated pan or are you building a traditional tile shower? When you say "double Threshold," do you mean it's finished on both sides? You can fabricate what you need out of bricks and mason mix...then tile over it. I like to have a piece of granite cut and polished for the job. No grout lines and very strong. As to the height, the top of the curb should be a minimum of 2" higher than the drain.

I agree, but was going to also suggest that the marble continue up the wall beyond the cap of the kneewall in order to also serve to cover the interior tile edge (so a bullnose is not necessary). The other option would be to use double row of bullnose up the wall beyond the kneewall cap, but the difference in material might be jarring. There are some challenges to the configuration that make some of the standards hard to employ in this scenario. We would typically see the floor material used on the front of the curb, but I think with the waterfall of the marble material, I would probably use that marble on the small amount of curb that is left. Much will be determined by what pieces are available in the line of tile that you selected. A really good tile installation professional can help you work through the details - and the devil is in the details on tile. Lot of good ideas by going up to PHOTOS tab. Select bathroom and then put kneewall in the search bar. Drop down box will appear and select the one that says "kneewall in bathroom". No one of them may do the trick, it may end up being a combination of ideas.

Contact Dulles Glass (https://www.dullesglassandmirror.com/). I had exactly the same situation as you (same size Kohler cast iron shower base in an alcove) and did not want holes drilled into the shower base. DG measured, built, and installed a sliding door (one side fixed, the other slides) without drilling into the shower base. Drilling through the tile was required to the hold the fixed wall and top rail. I paid $2025 after negotiation, yeah not cheap. For a DIY job, I had contacted Dreamline (sold at Home Depot) which told me that the following models don't require drilling into the shower base: Model: SHDR-1260728-01 Duet 56-60 in. W x 72 in. H Semi-Frameless Bypass Sliding Shower Model: SHDR-0960720-01 Infinity-Z 56-60 in. W x 72 in. H Semi-Frameless Sliding Shower Door Model: SHDR-1160726-01 Visions 56-60 in. W x 72 in. H Semi-Frameless Sliding Shower Door Here's a great video about installing a shower door. Note that he does NOT drill holes into the shower base. He's installing a Nadoli shower door. I don't know if that brand is available in the US (the presenter is in Canada).

The 'premium', as they call it, is a better material with more choices in size and color. The pan is treated as any other pan/base with the tiled walls coming down over the edges. I'm having the walls tiled, just don't want tile on the floor. The preformed bases are made with a slope and a drain location of the end, center or a grate-type drain along one end. Here is an example.