ᐅ Repainting the wall

Hello everyone!

I will try to keep this as brief as possible:

I am currently repainting one of my rooms and want to make sure I do everything correctly this time. I noticed beforehand that the paint on one wall started to peel off, so I used a putty knife to remove the old layers of paint along with, I assume, some plaster, as much as I could. I left the rest of the paint intact since it still seems to be adhering well there. I should mention that the affected wall is probably drywall. A white (slightly gray-tinted) emulsion paint will be applied to the walls afterward.

My questions are:

1. What is the best way to treat the bare spot (plaster, primer, sealer, etc.)?
2. Should I apply putty to the entire wall, or can the transitions between the drywall and the old paint layer be blended?
3. How should I handle the other two walls near the door area (is just an acrylic-based primer sufficient)?
4. How can I create a smooth transition between the left wall and the drywall (a gap appeared while scraping that I am now trying to fill)?

Thank you in advance for any expert advice!

To better illustrate my situation, here are some photos (click to enlarge):