Re-do paint over plaster in bathroom: How to feather/match?

I want to understand what went wrong, and how to repair it:

This bathroom in a loft was done in 2012. The plasterwork is really good — smooth and shiny; but the last year or so the paint (Dulux Trade, 2-3coats) has been peeling off. It has all been done by builders that have wrecked a series of things, so it hardly surprises me [their plastering subcontractor was great].

This is how much I can easily remove with a flat 4inch scraping blade, it peeled in large smooth flaps getting smaller and more brittle towards the edge.

I guess it should have been Bathroom paint, but I know they first did a 'mist coat'. So how do I match the white & chocolate areas? Should I scrape a straight edge with a knife or so all around the peeling area and then scrape all paint off up to that line (so the patch is a rectangle or near enough)? Or just remove the 'island' (caused by a repair, as they put the ventilator there originally).

Should I sand down the sharp edges now, then re-paint 2-3layers, then sand down the transition again, and then give the whole room a layer?

I suspect the paint higher up where the ceiling flattens off is also loose (because the same surfaces and products, and the hot moist air pools up there around ineffective ventilator).

Looking at the other 'peeling paint' questions, this is a much higher-moisture situation. And I'm guessing PVA-sealing doesn't do anything, because of the high PVA content of the plaster (UK standard practice) or does it still?