I saw the 16" circ listed but didn't see where you're to switch to using it. I guess when the number of stitches makes it hard to use the dpn. Like I said above, I'd just start and finish on the same circ. To each their own, I have np with someone else doing it differently. It does look like a fun knit, I really think I must do one.
You can't knit the left sock first. Ever.