We are going to have picture frame wainscoting put in our bathroom, so I've been researching it. Looking at your pictures, I would not do it in your bathroom. You will have major problems with the bullnose corners and especially the curved wall. I can't even imagine doing it. If it were my room, I would paint over the faux painting to give the room consistency, it's too chopped up with only certain walls treated. Especially since there's no clear cut ending to the faux because of all the curved corners. Maybe a nice, long tapestry on the long wall to the left of the toilet. Maybe get someone to paint a mural on the curved wall, since hanging anything on it might be a challenge. Good luck!

I would definitely expect there to be vertical frame members at every corner, with the size of each box varying to accommodate different wall lengths. In fact, I would expect there to be a vertical frame member on each side of each corner. The alternative of chopping up the horizontals to maintain a frame size that is unrelated to the dimensions of the room would strike me as silly. This is not wallpaper.

I'm not sure why it would be more of a fire hazard than a lot of other things that are in the wall, if you're working with fixtures/wiring that's all new. But then there are a number of things about code that I don't quite understand.

You will need to decide if you can live with different sizes or if you need to go with a pro to install custom. It seems like there are lots of sizes maybe you just need to sitdown and see if you can figure out a layout IMO I wouldn’t do it with those windows.