Let's see.. first, yes, you can do each wall in two pieces, with the butt edges against the wall and the tapered (smooth edged) sides making the joint. Or, if you want to avoid a seam, they're both small enough to do vertically, it looks like. It's easier to hang two pieces, though. Do the upper piece first, using the full 4' height, then cut the lower piece on the bottom as needed.
To cut out for the boxes, you can take good measurements to the sides of each and mark them out on the sheet, then cut out the hole with a knife before you hang the sheet. The lazy man's way that most drywall guys are using these days is to roughly mark the center of each box, hold the sheet against the wall, then use a rotozip tool with a drywall bit to punch the middle of the box, then cut around the edges.
You might be able to slide a sheet behind the existing corner bead, but the usual way would be to score along it on the "good wall, remove it, and reinstall fresh corner bead. You'd have to paint at least the edge of the adjoining wall, obviously.
Finally, the thickness of the existing drywall only matters if you need to blend a joint on a flat wall, which is not the case from the pictures you've shown. Use 1/2", it's easier.