Is that 12' peaked ceiling height, or do you just have very high ceilings? You'll need two coats, and depending upon the current finish and age it can vary on coverage. 2 gal would be a good start. If the old finish is very dry you may need up to 3.

To clarify, if I stood in the shower in your picture and looked up at the ceiling, I'd be looking at the sloped ceiling. The glass and the sheet rock are flat. So does the tall part of the slope start above the outward visual part of the sheetrock and glass? So is there a portion of the ceiling that is tiled down the side in the shape of a long, low triangle?

Gimbals will not move enough to meet the pitch of your ceiling unless the pitch is quite low. Your installation requires downlights that match the angle of your ceiling. Take a look at this spec sheet. Remodeling will require a different housing than new construction. https://s2.img-b.com/build.com/mediabase/specifications/elco/495187/elco-el616k-specification-sheet.pdf I recommend you select a housing that takes a regular medium base screw-in bulb. Then you can later decide what type of bulb you wish to use in it to get the quality of light you desire.

My DD has similar situation in one of her bathrooms- vanity against side of dormer. When she bought the house, it had entire area above vanity mirrored with picture frame mounding around it. She remodeled and took that mirror down, put new vanity in same (and only possible) position, and hung an unusual shaped mirror from Ballard above the vanity. She thought mirror would look hung low but worked perfectly. Think you just need to find right mirror if you don’t want to or are unable to rearrange plumbing.