Alice, you are working too hard. All you have to do is sand the edges where the paint is sticking to blend those areas into the bare plaster. A paint stripper will be a disaster since plaster is porous. The stripper will soak into the plaster and then work its way out after you have repainted. You might want to put a little spackle into the dings from the putty knife or whatever you were using to scrape and sand that smooth. Then prime with a good quality prime like Kilz. Follow with 2 coats of a good quality paint. I would choose a paint from a specialty paint store over that from a mass marketer. When the paint is sold directly by the paint manufacturer, it tends to be higher quality than when they are making it for someone else and price is a big issue.
Good luck and have fun with your project. :o) Henry