I’ve knitted two sleeves 17" long. They were knitted flat, tapered out, and raglan shaped and I will seam them to the other pieces. Unfortunately they are too tight and I need to widen them over the entire length.
Is the only/best way to do this is to knit a strip to insert between the two sides leaving a sleeve with two seams?
If the yarn is wool they may block bigger. Other than that you might have do as you suggested or frog and start over.
Those are the only alternatives I can think of. You may need to chack the armhole opening especially any bind off at the underarm to make sure that the wider sleeve will fit into it. it’s so painful when this happens, I know from experience but it’s worth getting this very good looking sweater right.