Background: I bought my first home recently from an elderly couple who had owned their home since 1970. The husband was a flooring contractor. In addition to installing different flooring in each of the ~12 rooms in the house, he sometimes also installed linoleum flooring on the walls. I want to remove the linoleum and paint the walls so they look normal.
After many hours using a hammer, scraper and occasionally a heat gun, I've removed the linoleum from the walls in two rooms. Now I'm receiving conflicting advice on what to do. The walls are in most places covered uniformly with a layer of glue. A handyman I work with says that we can cover it with drywall mud, sand it and then paint over it. My uncle who is an architect says I need to remove the glue first to make sure the drywall mud bonds to the wall. I know nothing about any of this. Do I actually need to remove all of the glue? If so, how do I do that? I tried using a sander, googone+wire brush, and a metal scraper. None of these worked well. I could use the heat gun to melt small portions of the wall and remove but it would take a really long time. If it helps, the walls are plaster.
Here are some pictures of the rooms. In some areas, you'll see that glue is chipping. In most areas, it is uniformly on there and will not come down without melting it or sanding it away or something.
Please do feel free to talk to me like I'm a five year old in the answers. I am new to home improvement and have no idea what I'm doing :)