Bathroom problems- mist coat on old plaster and peeling paint or seal with a primer?

Scrape off all loose/ flaking paint. Feather off edges of the paint left. Prime surface with a suitable primer (I would recommend Zinsser). Finish off with 2 coats of Dulux Trade diamond eggshell (or if preferred - Diamond Matt, although it’s not as good as eggshell).

Now it's best to remove all loses paint and prime bare plaster hope you get it sorted

As you already started, in order to get desired results you will have to remove the old paint all the way to the bare plaster. Some paint removers work, some don't, I would use a sander to remove the paint that doesn't come off with the scraper (remember PPE). Prior to painting please use a soft bristle brush to get most of the dust off the surface. Then use primer paint, I would say at least 2 coats to have a piece of mind. Then you would need to use paint that is specific for the bathroom, is mostly water and mould resistant. Also, a good practise is to make sure your bathroom have a good air circulation. If you have a fan, make sure that is clean from dust. If you do not have a fan, after shower or bath please leave either the window or the door wide open so the moisture in the air can escape.

Seems almost the entire ceiling need to be re worked. Peel of paint entirely, prime first then repaint

Rub down etc seal it with an oil based undercoat or pva, fill bad areas line walls with a 1400 lining paper before papering rub filler down seal walls with pva.then paint.