My walls have started crumbling

When you have sanded the walls back seal them then with Zinnser gardz this with take care if the powering. You can then fill them and repaint . You may have put the paint on while slightly damp, or you may not of thinned down the mist coat enough.

Did you use silk or satin paint? If so you have trapped moisture behind it which will cause a bad reaction,speak to your plaster with out seeing it it’s hard to say what’s gone wrong

Could be a few different issues here. What type of paint did you use for the feature wall? Have any of the other walls been effected with the new plaster paint ? The only way forwards or how I would do it would to Be scrape off any loose or flaking paint, Lightly sand the edges where the patches are. Apply Zinsser Bin or Zinsser Peal stop to entire wall( you might want to use a paint brush over the patches or the roller might lift more paint off) Once dry fill the patches, touch up with Zinsser over the filler then apply your paint over that.

As to the amount of time you waited, was the house kept warm during the drying process? If the walls were damp then this can lead to paint peeling and cracking off in sections. Also, if the walls were'nt sound below the skim, or bonded properly, then the plaster may come away in blown patches which would require hacking off and recoating. Hope that's some help to you