Question about pattern

[Dizzyisabelle]

Hi all!
I am knitting the Knock Around Hoodie from the Chicks with Sticks Guide to Knitting. I am currently working the back panel (first piece) and I have a few questions about the pattern.
First issue: the back is knitted on size 13 needles but then for the front and sleeves it says use size 10 1/2 needles, even though there is no mention of that in the materials list, nor is there an accommodation for the gauge switch in the pattern, so I am wondering of this is an error.

Secondly, I am at a point in the pattern where I've worked 42 rows of stockinette stitch and I am about to start raglan shaping. The directions say:
Bind off 4 sts at beg of next 2 rows. (got that - let's call those rows 43 and 44, which would be a knit row and then a purl row)
Then:
Dec 1 st (k2tog) at beg and end of every 3rd row 6 times. Then dec 1 st at beg and end of every 2nd row 7 times.
I've written all of the steps out row by row so I can check them off as I'm done with them.
My questions are - am I starting the decrease at row 45, a knit row, (immediately after the two bind off rows)? if yes, then I would do another decrease on row 48, correct? If that is the case, row 48 is a purl row, so rather than k2tog, would it not make more sense to p2tog?
I have read that most decreases are usually done on the same side, but every 3rd row is going to wind up on a purl side at some point, so I wanted to be sure I wasn't interpreting it wrong or reading too much into it.
Thank so much in advance!