It already has gyproc - which looks badly done. I would be tempted to cut a hole in it to see what is behind
Fill and bodge is not really the way to go. Overboard is best option. I've tried to remove tapes and re tape joints but it bubbles and looks naff.
Trick is with all such dodgy surfaces is to double skim , PVA and skim one coat with multi putting tape in affected area, flatten but trowel as little as poss Let dry PVA and skim properly A lot of fuss it may seem but am just doing such a job , bonded artex ceiling and dodgy walls The initial skim took 2 hr for the whole room
I used to use oil based undercoat. Remove tapes. Undercoat then get the hell out because it stinks. Go back next day apply neat Pva and skim. Prefabs built 1950s by me are all naff and like pic. Was terrible and frustrating work with bubbles and pealing. I was young and needed the money, but the undercoat seems to hold back the wet plaster and stop the problems... Almost
@DIY womble I can see that working to as first coat hardens off. I do remember getting a staple gun out in frustration and stapling the bubbles back. Was 28 years ago and I was a kid on £45 a day and it was cit work.. Don't tell anyone though.Shush
This room today was skimmed 3 days ago , some areas were bonded out as much as 10 mm, sockets moved about so old holes to fill , the customer had stripped the wallpaper as best he could so you can imagine the tears and gouges , very wavey ceiling , nice gloss paint on some patches so a good example of when to use this double skim method . I spent 2 hours wiping the room the first time and jolly pleased I did , just popped back to skim the ceiling for real and was a pleasure to do , easy peasey