I'm installing 5/8' underlayment over a 1/2" subfloor. I've seen
contradicting info on how to orient the underlayment. Some say right
angles to the subfloor, but some say right angles to the joists. I
cant have it both ways, since my subfloor was installed at right
angles to the joists. What to do? My previous underlayment (which I
removed) was laid across the joists so unless I can find a compelling
reason I guess I'll redo it that way.

Also, I want to use nails. Screws are just plain out of the question,
and I dont want the expense of renting or buying staple equipment. But
the shortest ring shank underlayment nails are 1 1/4", which means
every one will be punching a hole and splintering out the bottom of my
subfloor, which is not desirable. I know they make 25 mm (1 inch)
underlayment nails but I cant find them in the US (or Canada, so
far)..

What to do?