I understand that you want to remove the damaged material but you also don't want to scrape so aggressively that you end up removing a large amount of undamaged material that could have stayed in place, and it is difficult to know where to draw the line.
A good rule of thumb is that if the material can be removed using light pressure with just your fingers and crumbles away easily then it is too damaged that it probably shouldn't stay there.
Keep removing material that you are able to easily remove using only your fingers. You can use scraping tools to make the process easier and to save damaging your fingers from repetitive wear, but don't remove a piece of material using the scraper that you wouldn't have been able to remove using only your fingers.
You may find that there is material which can be easily removed, but hasn't been water damaged, particularly if this is an old house there may be crumbly plaster just due to it's age or pre-existing damage. Unfortunately that should be removed too.
Once the only material remaining is material which is strong enough to stay there that can't be removed by pulling with finger strength, you can plaster over it, but ensure it is fully dried out first and remove any dust from the surface with a brush.
The reason the wall hasn't dried out for a month is probably that the paint on the surface is preventing the moisture from escaping. Once the damaged paint is peeled off you can use a dehumidifier in the room for a few days with the windows and doors closed to help dry it out.
If you don't have a dehumidifier then the process will take longer but you can help by leaving a window slightly ajar.
I have successfully used this method to repair a plastered wall which was crumbling in parts due to it being over 100 years old. The part of a few square metres which I re plastered has held for over a year now.
EDIT: It looks like you may have to take down your curtain rail temporarily while you sort this out if the damage extends that far.
