To slip 1 purlwise (sl 1p), insert the right needle as if to purl and then just move the stitch to the right needle. If you click the GLOSSARY button at the top of the page and scroll down you can find the entry and choose which video you wish to watch. The number of DPNs you use won't really effect the way the heel is worked. Except for the stitches that you use to create the heel flap, the rest will just hang around and wait patiently for you to get back to them.
You can't knit the left sock first. Ever.