I plan to use Durock backer board as underlayment for tile (in
conjunction with plywood). My preference is to use 1/2". However, I
downloaded Durock's install guide and got confused. It sounded as if
they made two kinds, one for walls and one specifically for
underlayment. They call the 1/2" and 5/8" board their "Standard Board"
while the "Underlayment" version comes only in 5/16". Is there
anything that makes the 1/2" board unsuitable for underlayment or is
this just a matter of advertising the 5/16" as underlayment because
it's thinner and will not raise the floor as much?

Secondly, convention suggests a gap of 1/8" between boards. However I
can't seem to get a straight answer regarding the space that should be
left between the backer board and the wall. 1/8"? 1/4"?

Help is a appreciated!