I found enough courage to finally cut one of the arm steeks and pickup the sleeve stitches.
In the picture on the right, you’ll see the inside (private side) of the sweater and I’ve pointed out where I picked up stitches from the cut steek. So far the edge hasn’t unraveled much despite several attempts at picking up stitches before getting it right. When the sleeves are finished I’ll trim the steek stitches if necessary and tack down the remaining stitches as described in A. Starmore’s book.
In the picture on the left you’ll see the outside (public side) of the same sleeve. Notice how it starts in the middle of a lengthwise repeat. I’ve read that traditional Fair Isle sweaters are usually designed so that there are no partial lengthwise repeats. With that being said, I think the long single lengthwise repeat in this non-traditional sweater makes it impossible to avoid a partial repeat on the sleeves, especially the adult versions. I did however notice that the child’s version shown on the Virtual Yarn website, has sleeves with three full lengthwise repeats.