I have been in the trade for over 10 years and covered many different jobs big and small.
Hi did you notice a bulge before it was plastered? if not the plasterer may not of pva the ceiling properly before it was skimmed,as it will peel off.
Hi,you can over tack the ceiling with plasterboard once you locate the joists above a lot easier quicker and a lot less mess.scimming new board is always easier and it’s a better finish
My advice would be to scrape off as much as possible,it sounds like they didn’t pva the ceilings properly to create a bond to skim to,also i tend to was the ceiling before i pva to lose excess dust.if it wasn’t sealed the plaster will flake off....really need to start again