Field's Fabrics White - 1/2" Fusible Knit Stay Tape - 0.5" X 25 Yards SewkeysE Extremely Fine Knit Interfacing Sold by The 25 Yard Roll (KST-01) M494.09

So you decided to try it! With your arms down I see rippling on one side, but not on the other. Maybe you handled them differently, or maybe the seam you closed the addition with was under more stress? Bias stretch in seams can be such a bugger. I would try Sewkeyse in the silk side of the seams to stabilize them and gently reinsert the panel. Sewkeyse is extremely light weight and will unfuse from your fabric easily without damaging it: https://www.amazon.com/White-Fusible-SewkeysE-Extremely-Interfacing/dp/B07JJKR9H4?dib=eyJ2IjoiMSJ9.pDItHyCrgXCY--Dr86fDMmN9lzKjVDVBilDLj https://www.amazon.com/White-SewkeysE-Extremely-Fusible-M494-11/dp/B07JJYSK47?dib=eyJ2IjoiMSJ9.pDItHyCrgXCY--Dr86fDMmN9lzKjVDVBilDLjQ I'm not sure which would work best so I'd probably get both, though I do use the knit stuff on everything, and it has many uses. Also, I'd stitch from the point downward on both sides, with the addition on top since it won't be stretched as much by the advancing upper feed of the presser foot in the way that the silk will. And with the silk on the bottom, you might want tissue to protect it from the feed dogs. Hope this works because the panel really seems to have resolved your fit issues.