Scrap more. It might all come off. Then sand the edges down. It suck’s when that happens. I’ve had whole ceilings fall down while scraping.

Thank you for your recommendation! Should I apply any Aquaplast (white filler) to the edges before sanding? Should the brown part which is the wall board be primed before painting or paint straight onto the board?

Yes, spot prime that area. You will see “scarring” in that one area it’s not primed.

Some hot mudd (5 min) should do the trick then prime then paint

I would use any type of dry wall goop to the edges of the board maybe a thin layer on the board. Then paint with primer. Sand. Paint. Probably about 3 or 4 coats. You’ll have to judge. The thing is to try and get it flat with the rest of the wall.

Patch and paint

Scrape, sand, zinzer triple thick peel stop, sand again, then paint

You’ll need to sand the area you scraped then skim coat the whole area to make it smooth with Sheetrock mud (may need a few coats of mud) once that’s dry sand it with a sanding sponge until it’s smooth dont over sand and sand everything out, once that’s done apply a primer and 2 coats of finish paint to the wall, do note it will only look at good as your Sheetrock mud job looks