If you're going to knit the cardigan in the round, then you would definitely have to do steeks and
you would have to do intarsia in the round as well. I did a search on YouTube for intarsia in the round and found these
. The method for in the round is a bit different than when worked flat--that is, what's in the KH video, so do take a look at some of them before you jump in.
Personally, I would work this as one piece but not in the round; with this method, you still wouldn't have any side seams--just shoulder seams, but you'd have those with steeks, too. Knitting it flat as one piece is easy enough to do, and you could include the different colored button bands as you go using regular old intarsia like what's in the KH video.
The ribbing can be worked as a stripe, and the pocket can be knit at the same time as the ribbing (based on what I see in the picture). The collar can be added later in any color you choose.