Need help please asap...patching plaster pics attached
If there is any moisture whatsoever coming through the brick walls, whatever you put on the walls (paint, plaster, etc.) will be damaged in short order from the moisture, and all your time and money will be wasted.
I am confused by how the plaster is currently installed from your pictures and description. If it is coming off of the wall, then it will continue to be a problem moving forward. The wallpaper may have been the only thing holding it together. I wouldn't invest a cent in making improvements on top of a patch job of that plaster because it is likely to continue degrading even if you were able to make the patch job look halfway decent.
If *I* was going to spend any money on redecorating that room, I would forget about skimming the old crumbling plaster, and I would float new drywall (sheetrock) in the room instead.
Take all the plaster off of all the walls, clean the brick and make sure it is sealed. Frame the entire room w/ 2x4 's (framed flat so that the new surface frame was 2" off the brick) spaced 24" apart at the center. Then, float sheet rock on the new frame, mud it, and you're ready to redecorate (no need for skimming because you have a brand new wall surface to work with). Believe it or not, it's a pretty easy and fast job, and could be done very affordably by a pro as long as we're talking about a standard room dimension. Something in the neighborhood of a 12x14 room w/ 8ft ceilings could be done for about $500 plus materials where I live (new orleans), and it will last.
If the brick wall is clean, completely free of of moisture, and you are dead set on doing a patch job, you can patch the holes with drywall/sheetrock. It will be difficult to make it look good, however, because of the irregular shapes of the areas you intend to patch. Drywall can be cut with a box cutter, but it takes some practice to get good at it. There will be gaps that are easy to conceal if you are a good drywall floater, but very difficult if you don't know what you're doing. You will need to shim the drywall off of the brick using something along the lines of strips of plywood or 1x2"s. You can probably get the stuff to adhere to the brick wall (provided that it is a clean, dust- and moisture-free surface) using silicone caulking.