Can you put drywall on top of drywall? You can but in order to make it look right you should pull the window and door trim and add jamb extensions before you reinstall it. Similarly you should consider adding electrical box extensions.
When thinking about joint compound or spackle for cracks, the rule of thumb is to use spackle on small cracks or nail holes. However, if you get larger cracks or holes in the wall, you will need drywall compound and drywall tape to patch the hole.
Generally speaking, if the damage to your drywall is minor, such as a small hole or crack, you can probably repair it yourself. However, it is best to hire a professional for more significant damage.
Matte paint is the best option for hiding any imperfections that may have occurred during the drywall finishing process. If matte paint isn't available for you, try using a darker shade of paint. The darker colour will limit the light reflection in the room, so the seam isn't so apparent.