That's why we use fine crushed stone almost like a peastone but more angular. What it is that this stone is considered to be trap rock, just fine trap rock. Very easy to work with and there is no dust variable that will hold moisture or settle as much as screenings. It's clean and sizes range from about 1/8 to 1/4 . Even if NCMA does allow for use of concrete sand, I don't like it because its round and tends to move more and wash faster than trap rock.
I have being using 1/4" inch for quite sometime now and it works really good I use it for patios and walkways as well.
Before we use to use sand or stone dust mixed with portland to avoid any sand migration but now it's only 1/4" or even 3/8"
As far as keeping walls straight just use a string and the next rows have to be somewhat straight.
If you try to use the string on every course then you end up not aligning the blocks with the pins lock up 100% and even you have a straight wall overtime the force coming from behind will push that wall out, in the best case scenario it will align the block to the ping worse it will force the block out and failure will start at that point.
from time time to time some rows will vary they will not stay as the base row but unless the difference is a lot we fix misalignment once we install the coping.
PS by he record I don't care what anyone will say about using it for patios and walkways this topic has being discussed several times so let's not start another bedding material tread.