The extra space at the top of wall/edge of ceiling area is to reduce cracking due to building movement. Incidentally, the ceiling should be up first, going to the wall framing, and should end up resting on the wall sheet butted below it. The corner joint is held by the tape & mud and the extra distance to screws allows a little bit of flex before things start to rip/crack/tear as normal levels of framing movement occur.
This is established good practice for 30 years or more, but of course the building trades accept things so slowly that someone will be on momentarily to insist that they both must be screwed right to the edge, whatever research the Gypsum Panel Association completed decades ago be damned.
Ceilings have more screws because they are fighting gravity (and if there's insulation above, supporting more weight than just the panel). Walls simply need to be held in place, not held up. Screw schedules can also be reduced (or supplemented) by using adhesives.
1/2" from the edge is somewhat at risk of breaking out the edge of the panel, so you have to be careful; but if you are trying to make a butt-joint on 1.5" framing, 1/2" is pretty much what you need to both hit wood and hold the panel, though it's close on both counts. Where you have length rather than width of the framing to hit, coming back an inch or more from the edge is safer.
That is one reason I like putting 1x3 strapping on where practical - it's a much bigger target than a 1.5" edge. It also allows 12" support with 2 foot truss spacings for ceilings, and more structure to support modern levels of attic insulation, which can weigh quite a bit.