The Little Giants can be easily adjusted so that one end is shorter than the other- that way you can position it as an A on the stairs. I did this a few years ago outside on some stairs that lead to my basement when painting my house and it worked fine and was really solid. I'm going to try it inside over the winter.

These railings are custom. It's not like you can select parts and assemble them. Everything has to be welded. So if you are prepared for the $$$ of them contact a metal shop. Here are sources for staircase railing parts that can be ordered, but still have to be measured and assembled by someone. https://www.indital.com/ https://cheapstairparts.com/

Pondering your master bath again. Maybe the answer to shower entry is to have a free standing bath tub. It sure frees up floor space in both width & length directions over the typical “tub in a box” that the bigger tubs with tiled surrounds to support them take up. If you look at my pics, you’ll see one end of our slipper tub. it is a 6’ porcelain pedestal tub & not inexpensive - it was our splurge. But free standing tubs are popular & so many sizes, shapes & materials so all kinds of prices. just looking at your floor plan, imagining the space freed up with a free standing. A nice touch of luxury in a master bath too. We tiled our walls around the tub up to window height & the plumbing is in the narrow bump out under the window which I use as a shelf. You can see that in pic too.

I read the Q&A for NuWallpaper. As @User stated, the most important thing is the quality of the surface which must be smooth, painted and untextured. I'd put effort into preparing the surface: spackle, sand, paint ("pretty old beat up" isn't the right surface for success). If the wallpaper fails, it appears easy to remove and replace. Maybe set aside some extra in case that happens.