I started a pair of these and had the same problem!! What I did is use the Backwards Loop cast-on (http://www.knittinghelp.com/videos/cast-on
). So when you go down the row with your purls, purl until you can't purl anymore and then use this cast on method to add the appropriate stitches. Then once you get knitting again your tights will look like they have a bite taken out of them (a shape that looks like this "<" I believe it is for the heel slit.
As for the other question, if you want to work it backwards, I don't see why you couldn't. I wouldn't be confident enough to do it myself, but I'm sure other people who have made these might have done that