Adventures in Vintage Resales... fka adventures in baby-sewing

Ok, working slowly on the wrap dress below. The dress itself seems fine, except for needing to adjust the shoulders near the neck, basically flattening the shoulder. This is customary for me. It's the set in sleeves that are beating me up. I can not get enough ease to pull them up right on the shoulders. I have been checking the books, it seems there is not enough ease and recommends spreading the pattern up the center. I have only one piece left about the size of a fat quarter. If you were me, would you cut and spread and add a gusset/insert in the center and act like it should have been there? Please help!

Ok, I had previously shown this pattern and plans for it using coral linen and underlining it. This dress is now underway. After being sick, I had only managed the basting of the linen to the underlining. I do not have enough for shaped facings and nothing on hand that I liked. I have made continuous bias from the linen and plan on that instead. The sleeves use the hem lace. I have the sleeves put together waiting for hems, I have the shoulder seams and darts done. I have one side seam done. Around the kids today, that is a lot of progress. Clearly, it will not be done for tomorrow. I did v-neck the front neck, but may round that out some, the original was a pretty high neckline as are many vintage patterns. I will put lingerie straps to help keep it in place. I did check the fit today and it seems pretty good, considering I did add to the waist area and reduce the waist darts and lower/shorten the side bust darts. Funny, I should have enlarged the sleeve/bicep a bit. The...