Mist coating with textured masonry paint correction

From a professional point of view you should not be in anyway thinned or not be using masonry paint on any internal projects

If the paint has dried then yes you will need to sand it flat which could take some time. No easy fix I'm afraid. If the mist coat has taken then you should be able to paint on it once sanded flat. If you didn't thin the mist coat enough it might be possible to scrape it off but it sounds like you knew what you were doing regarding sealing the wall so I think you've made a bit of work for yourself.

Considering this is a mist coat you don't need to worry, if the wall needs sanding then do it. I use masonry mixed with emulsion to mist coat very dark walls (for example a couple of months ago I had to paint a dark purple wall jasmine, one mist coat mix masonry and emulsion brought it close enough to be covered in two coats). Key down the surface and apply your desired colour. With regards to the ceiling you may have just saved yourself a bit of work applying a second coat of white paint.