OK. Cast off is the same as bind-off, yes.
RE: the 2nd ball of yarn, it's having you do that so you can do both sides of the neck at once. Is this a V-neck? Sounds like it wants you to divide so you can make both sides of a V. You're using your original ball of yarn for the side where your working yarn is when you divide, and a separate ball for the other side. This allows you to knit both sides at the same time, on the same needle.
As for armholes and neck at the same time, you'll start a RS row with an armhole edge. You'll do your armhole shaping here, then work across the row to the middle, where you divided for your neck. You'll do your neck decreases here, on each side of the divide. You'll continue to work across the row over to the other armhole, where you'll do your armhole shaping again.