After swatching with a superwash yarn and seeing a marked difference in the stitch quality, I decided that there are two keys to success with Stella. You need a yarn (or yarns) with good loft and, as other ravelers have suggested, you need to use ssk\ssp instead of K2tog\P2tog on the pattern biasing to the left.
Modifications:

1. I’m a loose knitter, so I used #8 as my primary needle. However, on the sleeves and lower body, I used a # 10 for the first 5 repeats to emphasize the flare. On the sleeves only, I went back to #9 for one repeat and then #8s up to the last 3 repeats, where I used a #9 to accommodate my larger arms. I went down to a #6 for the collar.
2. Before I joined the sleeves and lower body for the yoke, I blocked them thoroughly, including pinning out the points. I simply left the lower body on the needles and I put the sleeves on waste yarn. You can see the difference between the body and the yoke, even though I dampened and pinned the yoke and collar after the entire piece was done.
3. I used a regular bind-off instead of the decreasing bind-off for the top and collar as I thought the decreasing bind-off was too bulky.

I love, love, love this unique and creative design!