We have an article here
with some helpful tips. The article was originaly intended for FW
tanks, but has many parallels to moving a SW
I'm not sure I completely follow your first question.. are you asking that if you put existing substrate on top of new substrate, how long before the water will be safe for corals?
hard to tell, if your move doesn't kick off a new cycle then the water is safe as soon as your sure your not in another cycle and your tests say the water is ok.
I don't have any experience with acros so i can't answer your second question, but I would say the shorter the better. If it's possible, can you set up your tank lights to hang over your temporary containers? I would if you could.