My best advice is to remove the insulation and vapor barrier. Add a larger piece of lumber to the side of each existing rafter. This should be deep enough to allow a minimum air space of 2.5 inches plus the thickness of the desired insulation. Styrofoam baffles should be used from the wall to the ridge to prevent insulation from being pushed in to far. Finally adequate soffit venting and a ridge vent should be installed to provide the proper air flow.
We agree, ceiling texture should not be used in kitchens and bathrooms. However, it happens! There are two types of texture, popcorn and plaster. They can both be removed by soaking with water and scraping with a putty knife, providing they have not been painted over. We recommend that you do a test spot to see how (or if) it will come off.
Your best bet may be to just scrape off the infected area and have it re-stippled. I believe you can get spray-can's of stipple for small patches. The best way to remove the infected area is to wet it and scrape it with a putty knife. You may have to re-paint the entire ceiling afterwards, as the paint (assuming it's white) may have faded or discolored over time. Just be sure to use a roller specifically for the job. Any paint store can help you out with stipple and paint for the ceiling.
We've install numerous tiled steam showers and have never sloped a ceiling nor heard any type of complaint regarding dripping. You could slope the ceiling however this may cause more difficulty installing the frameless glass as well as experience a less attractive finish.
If your ceiling is only stained, you can paint it with an oil based paint. If it is damaged slightly, you can apply a skim coat of mud, sand it and apply oil paint. If it is badly damaged, you may have to cut out the existing drywall and install a new piece, then tape/mud/sand and paint.
Unfortunately, the ceiling clean up is labor intensive. Generally water based which can be softened with water and a sponge. Scraper required and elbow grease. You will probably encounter some repair after also and will have to fill and sand as required. Then a proper sealer will have to go on to prevent further oxidization bleeding through.
Most wallpapers can take paint, however test in a location that is not obvious. That is if you want to keep the texture that the paper provides. However if it is a soluble paper it can be removed with a wet sponge and scraper. You may want to paint a texture on the large walls to create more warmth to the space.
Scraping is normal. If you have painted it at all it will be harder. If a latex was used you can wet it with a sponge and water to soften areas as you go.
Beauty is in the eye of the beholder. Sounds like a good idea to me, be aware that in a small room such as a bathroom, several colors may make it too cluttered.
If you are talking about the stipple, it takes a sponge, water along with a scraper and plenty of elbow grease.